Lesson 10 - Light- Reflection and Refraction-Class 10 Science Notes

Light - Reflection and Refraction

Light:- 

Light is a form of energy that enables us to see things around us. 

Light starts from a source and bounces off from objects which are perceived by our eyes and our brain processes this signal, which eventually enables us to see.

Some common phenomena:- There occur some common wonderful phenomena associated with light are image formation by mirrors, twinkling of stars, beautiful colors of rainbow, bending of light by a medium.

Nature of Light:- 

Light behaves as-

• ray  (e.g. reflection and refraction)
• wave  (e.g. interference and diffraction)
• particle  (e.g. photoelectric effect)

Properties of Light:- 

• Light is an electromagnetic wave, so does not require any medium to travel.
• Light tends to travel in straight line.
• Light has dual nature i.e. wave as well as particle.
• Light casts shadow.
• Speed of light is maximum in vacuum. Its value is 3×10⁸ m/s.
• When light falls on a surface, following may happens-  

1. Reflection
2. Refraction
3. Absorption

How do we see any object?

An object reflects light that falls on it. This reflected light, when received by our eyes, enables us to see things.

Ray of Light:- When light travels from its source of light, it looks like straight line, this line is called a ray of light.

Shadow:- When a beam of light passes through an opaque object and gets reflected and it form a shadow of that opaque object.

Diffraction:- If an opaque object on the path of light becomes very small, light has a tendency to bend around it and not walk in a straight line. This effect is known as diffraction of light.

Types of objects:- 

There are two types of objects-

1. Luminous objects
2. Non- Luminous objects

Luminous objects:- 

Objects which have their own light are known as luminous objects. When their light reaches our eyes that is the time when we can see them.

Example- Sun, tube light etc.

        If any substance glows on passing current, that also falls under this category.

Non-Luminous objects:- 

Objects which do not have their own light are known as Non-Luminous objects. To see, we need a luminous object around it.

Example- Furniture, Walls, Moon etc.

Medium:- 

The material through which light exhibits its different phenomena is called medium. There are three types of medium based on the passage of light.

1. Transparent medium

2. Opaque medium

3. Translucent medium

1. Transparent medium:- The medium that allows light to pass through completely is called transparent medium.

Example- Air, Glass, Pure water etc.

2. Translucent medium:- The medium through which light rays can pass partially is called translucent medium.

Example- fog, Ground glass, etc.

3. Opaque medium:- The medium through which light rays cannot pass at all is called opaque medium.

Example- Metal, Wood, etc.

Reflection of Light:-

When a ray of light falls on polished surface, it turns or reflects most of the light in the same medium from which it comes. This phenomenon is known as reflection of light.

Terminology involved in case of reflection:-

1. Incident Ray- It is the ray that strikes the surface.
2. Reflected Ray- The ray that bounces back on striking the surface.
3. Normal- The perpendicular drawn to surface.
4. Point of Incidence- It is the point at which the incident ray strikes the surface.
5. Angle of Incidence- The angle between the incident ray and the normal.
6. Angle of reflection- The angle between the reflected ray and normal.
7. Plane of Reflection- Plane where incident ray and normal ray lies.

Characteristics of Best Reflector:-

There are many surfaces that show reflection but not all surfaces show reflection. There are certain characteristics that must be fulfilled-

• It should have a shiny surface.
• It should have a polished surface.
• It should have a smooth surface.

Out of all, Silver metal is the best reflector.

Law of reflection of light:- 

1. The angle of incidence is equal to the angle of  reflection.
2. The incident ray, the normal at the point of incidence and the reflected ray, all lie in the same plane.

These laws of reflection are applicable to all types of reflecting surfaces.

Type of Reflection:- 

• Specular or Regular reflection:- Such type of reflection takes place from shiny or polished surface and incident rays remain parallel to each other after reflection.

• Diffused or Irregular reflection:- Such type of reflection takes place from rough surface and incident rays do not remain parallel to each other after reflection.

Types of sources of light:- 

There are different objects that emit light, so depending upon the size of object, we have two types of sources of light-

1. Point sources
2. Finite sources

Point sources- These are those sources that are too small in size.

Finite sources- These are the sources that have certain height.

Mirror:-

This is a shiny and more polished reflecting surface, which make the image of an object kept front of it.

Types of mirror:- 

• Plane Mirror:- Its reflecting surface is straight and flat.

• Spherical Mirror:- It has curved reflecting surface. This curved reflecting surface can be curved inward or outward.

On the basis of curvature, spherical mirror are of two types-

• Concave Mirror:- Its reflecting surface is inward. It converges the light so it is also called converging mirror.

• Convex Mirror:- Its reflecting surface is outward. It diverges the light so it is also called diverging mirror.

Parts of spherical mirror:-

• Pole- The center of reflecting surface of spherical mirror is called its pole. It is denoted by English letter P.
• Centre of Curvature- The reflecting surface of  a spherical mirror forms from a part of a sphere. This sphere has a center. This point is called the center of curvature of the spherical mirror.
• Radius of curvature- The distance between Pole and center of curvature of the spherical mirror is known as the radius of curvature.
• Principal axis- The straight line passing through the pole and the center of curvature of spherical mirror is known as principal axis.
• Principal focus- There is another point F on the principal axis lie between half of the pole and the center of curvature which is called principal focus. Incident ray parallel to principal axis intersect at principal focus after reflection in concave mirror and it seems to intersect in convex mirror.
• Focal length- Distance between the pole and the principal focus of the mirror is called Focal length. It is denoted by small 'f ' of English alphabets. This distance is half of the radius of curvature.
• Aperture- The reflecting surface of a spherical mirror is by and large spherical. The surface, then, has a circular outline. The diameter of the reflecting surface of spherical mirror is called its aperture.

Image:-

An image is formed when the reflected ray actually meet or appear to meet at a certain point.

There are two types of images-

1. Real images
2. Virtual images

Real Image:-

1. It is the image which is formed when reflected rays actually meet at a certain point.
2. It is always inverted.
3. It can be obtained on screen.
4. Example: Image formed on cinema screen and formed by concave mirror.

Virtual Image:-

1. It is the image which is formed when reflected rays do not meet actually but appear to meet at a certain point.
2. It is always erect.
3. It can't be obtained on a screen.
4. Example: Image formed by plane mirror or convex mirror.

The Position, Nature and Size of Image:

The position of object: The place where an object is placed.

The position of image: The place where mirror forms the image of an object.

The size of image: This is the size of image which tells that the image of object is formed either smaller or larger than the object.

The nature of image: It is known by the nature of image that what type of image is formed by mirror like virtual and erect or real and inverted.

There are two types of nature of images-

1. Real and inverted:- This type of images always form in front of mirror.
2. Virtual and erect:- This types of images always form behind the mirror.

Image formation by Plane mirror:-

In this many light rays are emitted by the object but in order to make a ray diagram, we consider at least two rays. We consider the two rays that are striking the mirror at different angles. Let's say, one striking at normal to mirror and the other at a certain angle. The normal ray retraces its path and the other ray striking at a certain angle is reflected by the same angle. When we produce both the reflected rays, they appear to meet at a certain point and there the image is formed.

Characteristic of image formed by plane mirror:-

1. Virtual
2. Erect
3. Same size
4. Distance of Object is equal to Distance of Image from mirror
5. Laterally inverted

Lateral Inversion:-

The right side of the object appears left side of the image and vice-versa.

The word AMBULANCE is written in reverse direction so that it can be read correctly in rear view mirror of vehicles going in front of it.

Rules for making ray diagram by spherical mirror:-

1. A ray parallel to the principal axis, after reflection, will pass through the principal focus in case of a concave mirror or appear to diverge from the principal focus in case of a convex mirror.
   
   
   
2. A ray passing through the principal focus of a concave mirror or a ray which is directed towards the principal focus of a convex mirror, after reflection, will emerge parallel to the principal axis.
   
   
   
3. A ray passing through the center of curvature of a concave mirror or directed in the direction of center of curvature of a convex mirror, after reflection, is reflected back along the same path.
   
   
   
4. A ray incident obliquely to the principal axis, towards a point P (pole of the mirror), on the concave mirror or a convex mirror, is reflected obliquely. The incident and reflected rays follow the laws of reflection at the point of incidence (point P), making equal angles with the principal axis.
   
   
   

Ray diagram for images formed by concave mirror:-

1. When object is at infinity

Image position - at 'F'

Nature of image - real and inverted

Size - point size or highly diminished

2. When object is beyond 'C'

Image position - between 'F' and 'C'

Nature of image -real and inverted

Size - diminished

3. When object is at 'C'

Image position - at 'C'

Nature of image - real and inverted

Size - same size as that of object

4. When object is placed between 'F' and 'C'

Image position - beyond 'C'

Nature of image - real and inverted

Size - enlarged

5. When object is placed at 'F'

Image position - at infinity

Nature of image -real and inverted

Size - highly enlarged

6. When object is between 'P' and 'F'

Image position - behind the mirror

Nature of image -virtual and erect

Size - enlarged

Uses of Concave mirror:-

1. Concave mirror are commonly used in torches, search lights and vehicles headlights to get powerful parallel beams of light.
2. They are often used as shaving mirrors to see a large image of the face.
3. Concave mirrors are used to see large images of the teeth of patients by the dentists.
4. Large concave mirrors are used to concentrate sunlight to produce heat in solar furnaces.

Ray diagram of images formed by convex mirror:-

1. When object is placed at infinity

Image position - at 'F'

Nature of image - virtual and erect

Size - point size

2. When object is placed between pole and infinity

Image position - between 'P' and 'F'

Nature of image - virtual and erect

Size - diminished

Uses of Convex mirror:-

1. Convex mirrors are commonly used as rear-view (wing) mirrors in vehicles.
2. It is used as reflector for street lighting purposes.
3. Convex mirrors are useful for inspecting places difficult to get to.
4. Convex mirrors are also used for security situation.

Sign convention for Reflection by Spherical Mirrors:-

Reflection of light by spherical mirror form a cartesian plane which is a set of sign conventions called the New Cartesian sign convention.

These are-

• The pole (P) of the mirror taken as the origin.
• The principal axis of the mirror is taken as the x-axis (XX') of the coordinate system.
• The object is always places to the left of the mirror. This implies that the light form the object falls on the mirror form the left-hand side.
• All the distances parallel to the principal axis are measured from the pole of mirror.
• All the distances measured to the right of the origin (along +x-axis) are taken as positive while those measured to the left of the origin (-x-axis) are taken as negative.
• All the distances measured perpendicular to and above the principal axis (along +y-axis) are taken as positive.
• Distances measured perpendicular to and below the principal axis (along - y-axis) are taken as negative.

u = negative

f =  negative

v =  positive

Mirror formula:- 

1/f =  1/u + 1/v

Where f = focal length

u = distance of object from pole

v =  distance of image from pole.

Magnification:-

Linear magnification is defined as the height of image to the height of the object. It is represented by 'm'.

m = Height of image/ Height of object

m = I/O

Also, m = -v/u

• If 'm' is negative, image is real.
• If 'm' is positive, image is virtual.
• If I=O, then m=1, i.e. image is equal to object.
• If I>O, then m>1, i.e. image is enlarged.
• If I<O, then m<1, i.e. image is diminished.

Note:- 

1. Magnification of plane mirror is always +1. '+' sign indicates virtual image.
2. If 'm' is '+ve' and less than 1, it is a convex mirror.
3. If 'm' is '-ve', it is a concave mirror.

Refraction

Refraction of Light:-

When a ray of light enters from one transparent medium to another transparent medium it changes its direction or it bends. This bending of light path is called refraction of  light.

The refraction of light only takes place through transparent materials like glass, air and water etc.

Situation of bending:-

There are two situation of this bending-

1. When a ray of light enters from rarer medium to denser medium it bends towards the normal.
2. When a ray of light enters from denser medium to rarer medium it bends away from normal.
   

Cause of refraction of light:-

The refraction of light occurs due to changing in speed of light when it enter on transparent medium to another.

Speed of light is maximum in vaccum. It is 3×10⁸ m/s

Law of refraction:- 

Following are two laws of refraction of light-

1. The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane.
2. The ratio of sine of angle of incidence to the sine of angle of refraction is a constant, for the light of a given color and for the given pair of media. 
   

Snell's law:-

The ratio of sine of angle of incidence to the sine of angle of refraction is a constant, for the light of a given color and for the given pair of media. This law is known as Snell's law of refraction.

sin i/sin r =  constant

Medium:-

The material through which light exhibits its different phenomena is called medium. There are two types of medium based on the refraction of light.

Denser medium:- The medium which has greater refractive index is called denser medium. The particles of this medium is denser.

Rarer medium:- The medium which has smaller refractive index is called rarer medium.

• When light enters obliquily from rarer medium to denser medium, it bends towards normal.
  
  
  
• When light enters obliquily from denser medium to rarer medium, it bends away form normal.
  

Refractive index:- 

It represents the amount of extent of bending of light when it passes from one medium to another.

There are two types of refractive index-

• Relative refractive index
• Absolute refractive index

Relative refractive index- Refractive index of medium with respect to other medium is called relative refractive index.

Absolute refractive index- Refractive index of medium with respect to air or vacuum is called absolute refractive index.

where c = 3 × 10 ⁸ m/s

Note:- 1

• The refractive index can be linked to an important physical quantity, the relative speed of propagation of light in different media. It turns out that light propagates with different speeds in different media. Light travels the fastest in vacuum with the highest speed of 3×10⁸ m/s.
• In air, the speed of light is only marginally less, compared to that in vacuum.
• It reduces considerably in glass or water. The value of the refractive index for a given pair of media depends upon the speed of light in the two media.
• Refractive index with higher value of material has less speed of light through it.
• Refractive index with lower value of material has high speed of light through it.

Note:- 2

Refractive index of one medium is reciprocal of refractive index of other medium given in pair.

Note:- 3 

Relation between relative refractive index and absolute refractive indices of two mediums.

   

Lens:-

A transparent refracting medium bounded by two surface, of which one or two surfaces are curved.

Types of lenses:-

There are mainly two types of lenses-

• Convex lens:- A lens with outward curve is called convex lens. Thickness at the center of convex lens is more than the thickness at the edges of lens. It converges the rays of light, so it is also called converging lense.
  

  

  
  
• Concave lens:- A lens with inward curve is called concave lens. Thickness at the center of concave lens is less than the thickness at the edges of lens. It diverges the rays of light, so it is also called diverging lens.
  

  

  
  

Note:- Lenses are classified into following types -

1. Simple lenses:- Simple lenses are basically classified in to eight types according to the curvature of the two optical surfaces. 

    a. Converging Lenses

• Convex Lens
• Biconvex lens
• Plano-convex lens
• Positive meniscus

    b. Diverging Lenses

• Concave lens
• Biconcave lens
• Plano-concave lens
• Negative meniscus

2. Combined lenses:- Combined lens generally refers to a lens made by combining two or more lenses, such as a convex lens that collects light and a concave lens that emits light. 

3. Aspherical Lens :- This lens is made of curved surface other than spherical surface. It is designed to reduce the number of lenses by minimizing spherical aberration and to bring the focus to one, resulting in a clean image with minimal distortion.

4. Cylindrical Lens :- Cylindrical lens focuses or magnifies light only within one dimensional single axis of the light source. These cylindrical lenses are used to focus light on thin lines in the field of optical metrology, laser scanning, spectroscopy, laser diodes and optical processor applications.

5. Chromatic Lens :- Achromatic lens has an excellent off-axis performance and designed to have a constant focal length regardless of aperture and operating wavelength.

Parts of  a lens:-

• Optical center (O):- The midpoint or the symmetric center of a spherical lens is known as its Optical Centre. It is also called the pole.
• Principal Axis:- The line passing through the optical center and the center of curvature.
• Centre of curvature (C):- The center of the spheres that the spherical lens was cut of. A spherical lens has two centers of curvature.
• Focus (F):- A point on principle axis at half of optical center and center of curvature. A spherical lens has two focus-  Primary focus(F1) and  Secondary focus(F2)
• Primary Focus (F1):- It is the point on the axis of a lens to which parallel rays of light converge or from which they appear to diverge after refraction.
• Focal length (f): Distance between optical center and focus.
• Radius of curvature (R):- Distance between optical center and radius of curvature.
• Aperture:- The diameter of refracting part of lens through which refraction occurs,  is called aperture. 
• Paraxial Ray:- A ray close to the principal axis and also parallel to it.

Principal Focus:- The principal focus of a concave lens is a point on its principal axis from which light rays parallel to the principal axis, appear to diverge after refraction by the lens.

• First Principal Focus (F1) : The first principal focus (F1) of a concave lens is the virtual position of a point object on the principal axis of the lens, for which the image formed by the concave lens is at infinity. It is usually denoted by the letter (F1).
•  Second Principal Focus (F2): The second principal focus (F2) of a concave lens is the position of image point on the principal axis of the lens, when the object is situated at infinity. It is generally denoted by the letter (F2).

Rules for making ray diagram by lens:-

1. A ray of light coming parallel to principal axis, after refraction through the lens, passes through or appears coming form the principal focus (F).
   
   
   
2. A ray of light passing through the optical centre O of the lens goes straight without suffering any deviation.
   
   
   
3. A ray of light coming from the object and passing or appearing to pass  through the principal focus of the lens after refraction through the lens, becomes parallel to the principal axis.

Ray diagram for images formed by convex lens:-

1. When object is at infinity

Image position - at 'F'

Nature of image - real and inverted

Size - point size or highly diminished

2. When object is beyond 'C'

Image position - between 'F' and 'C'

Nature of image -real and inverted

Size - diminished

3. When object is at 'C'

Image position - at 'C'

Nature of image - real and inverted

Size - same size as that of object

4. When object is placed between 'F' and 'C'

Image position - beyond 'C'

Nature of image - real and inverted

Size - enlarged

5. When object is placed at 'F'

Image position - at infinity

Nature of image -real and inverted

Size - highly enlarged

6. When object is between 'P' and 'F'

Image position - behind the mirror

Nature of image -virtual and erect

Size - enlarged

Uses of Convex lens:-

1. It is used in building microscopes.
2. Used as a magnifying glass.
3. used to adjust vision on spectacles
4. They are extensively employed in cameras to focus on and magnify an image.

Ray diagram of images formed by concave lens:-

1. When object is placed at infinity

Image position - at 'F'

Nature of image - virtual and erect

Size - point size

2. When object is placed between pole and infinity

Image position - between 'P' and 'F'

Nature of image - virtual and erect

Size - diminished

Uses of Concave lens:-

1. Peepholes(झरोखा) use a concave lens.
2. They are used to adjust vision on spectacles.
3. They are used to focus a camera on a single object.
4. These are utilized in flashlights as well.

Sign convention for Refraction by Lenses:-

Refraction of light by lens form a cartesian plane which is a set of sign conventions called the New Cartesian sign convention.

These are-

• The optical center (O) of the lens taken as the origin.
• The principal axis of the lens is taken as the x-axis (XX') of the coordinate system.
• The object is always places to the left of the lens. This implies that the light form the object falls on the lens form the left-hand side.
• All the distances parallel to the principal axis are measured from the pole of lens.
• All the distances measured to the right of the origin (along +x-axis) are taken as positive while those measured to the left of the origin (-x-axis) are taken as negative.
• All the distances measured perpendicular to and above the principal axis (along +y-axis) are taken as positive.
• Distances measured perpendicular to and below the principal axis (along - y-axis) are taken as negative.

u = negative

f =  negative

v =  positive

Lens formula:- 

1/f =  1/v - 1/u

Where f = focal length

u = distance of object from optical center

v =  distance of image from optical center.

Magnification:-

Linear magnification is defined as the height of image to the height of the object. It is represented by 'm'.

m = Height of image/ Height of object

m = I/O

Also, m = v/u

• If 'm' is negative, image is real.
• If 'm' is positive, image is virtual.
• If I=O, then m=1, i.e. image is equal to object.
• If I>O, then m>1, i.e. image is enlarged.
• If I<O, then m<1, i.e. image is diminished.

Note:-

1. If 'm' is '+ve' and less than 1, it is a concave lens.
2. If 'm' is '-ve', it is a convex lens.

Conclusion:-

To read this lesson in Hindi click here...

This page is under construction...