Light Q & A Class 8 Physics

What are the laws of reflection on plane surfaces?

When light rays are reflected by a smooth plane surface such as that of a plane mirror:

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

What do you mean by rectilinear propagation of light?

Rectilinear propagation of light is a property of light by which it travels in a straight line.

Describe an experiment to prove the laws of reflection?

Fix a white paper on a drawing board with the help of pins. Draw a straight line AB at the centre of the paper and a normal to AB at the point ‘O’ (ON). Draw a straight line PQ making a certain angle ‘i’ (say 45°) with ON. Fix a plane mirror strip vertically coinciding with AB. Fix two pins P & Q on the paper vertically and observe their images in the mirror. Now fix two more pins R & S such that they are in the same line as that of the images of P & Q in the mirror. Join R, S and, O. Measure the angle between RS & ON (r). You will find that i = r. Repeat the experiment with different angles of incidence and measure the corresponding angles of reflection. It will be observed that the angle of incidence will always be equal to the angle of reflection. Moreover, PQO (incident ray), ORS (reflected ray) and ON (normal) all lie in the same plane. This proves the laws of reflection.

Explain how an image forms in a plane mirror with the help of a diagram?

Consider a point object ‘O’ placed before a mirror PQ. Several light rays fall on the mirror and are reflected as shown in the diagram. The light ray ON is reflected along the same path. For other incident rays which fall obliquely, the reflected rays are divergent. When these rays fall on the retina of the eye, they appear to be coming from ‘I’ where the image is formed. Similarly, the light rays originating from a person’s face fall on the mirror and are reflected. The reflected rays reach the person’s eyes, and he can see his image in the mirror. (Diagram)

What is the nature of the image formed in a plane mirror?

The following are the characteristics of the image formed on a plane mirror:

1. It is a virtual image.
2. The image is erect or upright.
3. The image is laterally inverted.
4. Size of the image is equal to the size of the object.
5. The image distance is equal to the object distance.

What is a virtual image?

Images that cannot be caught on a screen are called virtual images. The eyes can only see such images.

What is the difference between a concave and convex surface?

If a hollow glass sphere is cut, it has two surfaces. (S1 & S2). The surface S1 that bulges outwards is called the convex surface, and the surface S2 that bulges inwards is called concave surface.

What are convex and concave mirrors?

In a spherical mirror:

1. If the convex surface is silvered, the concave surface as a reflecting surface and becomes a concave mirror.
2. If the concave surface is silvered, the convex surface acts as a reflecting surface and becomes a convex mirror.

Explain how an image is formed by a concave mirror with the help of a diagram?

Let APB be a concave mirror. Consider a light ray ‘X’ from an object travelling parallel to the principal axis and falling a point ‘A’ on the mirror. AC is normal to the surface at ‘A’. The angle XAC is the angle of incidence (i).  AY is the reflected ray, and angle CAY is the angle of reflection (r).  According to the laws of reflection, i = r. 

Now consider a parallel beam of light from the object parallel to the principal axis and incident on the concave mirror. All these light rays get reflected as above and pass through a point F. The image of the object is thus created in the plane passing through the point F and perpendicular to the principal axis. The point F is called the focus and the plane passing through it is called the focal plane.

Define Centre of curvature, Radius of curvature, Pole, Principal axis, Principal Focus, Focal length and, Focal plane?

1. Centre of curvature: The centre of the sphere of which a spherical mirror is a part of, is called the centre of curvature of that mirror.
2. The radius of the sphere of which a spherical mirror is a part of is called the radius of curvature of that mirror.
3. Pole: The geometric midpoint of a spherical mirror is called its pole.
4. Principal axis: When the radius of curvature of a spherical mirror is extended indefinitely on both sides, it forms the principal axis of that mirror.
5. Principal focus: It is that point on the principal axis to which all the rays originally parallel and close to the principal axis converge, or from which they appear to diverge, after refection from the mirror.
6. Focal length: The distance between the pole of a spherical mirror and its principal focus is called the focal length (f) of that mirror.
7. Focal plane: A plane passing through the principal focus of a spherical mirror and perpendicular to the principal axis is called its focal plane.

Describe an experiment to determine the focal length of a concave mirror?

Take a concave mirror (M) and fix it on a V-shaped wooden stand. Turn the wooden stand placed on a table towards an object at a large distance such as a tree or a light pole. Introduce w white screen and adjust its position until a bright clear image is formed on it. A parallel beam of light rays coming from the distant object after reflection in the mirror meets in the focal plane to form an image. The distance between the mirror and screen can be measured accurately to give the focal length of the mirror.

Explain how an image is formed by a convex mirror with the help of a diagram?

Consider a convex mirror APB. C is the centre of curvature, and F is the principal focus. Let an object XY be placed at a certain distance. The light ray travelling parallel to the principal axis is reflected as shown in the diagram and gets diverged. Another light ray falling at P is also reflected as per the laws of reflection. Both the reflected rays appear to becoming from the point I. Thus IZ is the virtual image of the object XY.

What is the nature of the image formed by spherical mirrors?

 Concave mirror

1. The image is real. (unless object distance < focal length)
2. Size of the image is less than the size of the object. (unless object distance < radius of curvature.
3. The image is inverted. (unless object distance < focal length)

Convex mirror

1. The image is always virtual.
2. Size of the image is always less than the size of the object.
3. The image is erect and diminished.

Explain the uses of Concave and Convex mirrors?

Concave mirror

1. They are used as reflectors in the headlights of vehicles. An intense parallel beam of light is obtained if the source of light (bulb) is placed at the focus, illuminating objects at a large distance.
2. A concave mirror forms a magnified image of an object placed at a location between its focus and centre of curvature. So dentists and ENT specialists use them to view the teeth, inner parts of throat, nose etc. It is also used as a shaving mirror.
3. They are also used in telescopes.

Convex mirror

1. Since it always gives a small and erect image, it is fitted in front of the driver of a vehicle so that he can get a clear rear view without having to turn backwards.
2. They are also used in telescopes.

Define refraction?

When light rays travel from one medium to another, they bend at the border between the two media due to a change in the velocity of light. This phenomenon is called refraction.

Describe an experiment to determine the refractive index of a glass slab?

Take a glass slab and put it at the centre of a white paper fixed to a drawing board.  Mark the edges with a pencil on the paper and name the rectangle ABCD. Take a point ‘O’ at the centre of AB and draw a perpendicular to AB at O. ON is the normal to AB. Draw a straight line PQ making a certain angle (say 30) with the normal. Fix two pins P and Q on that line. Observe the images of the pins P’ and Q’ through the surface CD and fix two more pins R and S all the pins appear on the same straight line. Join R and S with O’ on CD and join OO’. PQ represents the incident ray; OO’ represents the refracted ray. And RS represents the emergent ray. Measure the angles i and r, as shown in the diagram. Repeat the above with different values of i and measure r in each case. Determine the value of sin i and sin r and record them in a table. Calculate in each case. The value will be constant verifying the law of refraction.  Calculate its average, and this will be the refractive index of the glass slab.

What are the effects of refraction?

The following are the effects of refraction:

1. A coin appears to be raised when placed underwater.
2. Letters appear to be raised when viewed using a glass slab.
3. A rod appears to be bent when placed underwater.
4. Stars appear to twinkle due to refraction of light in air at different temperatures.
5. Sun appears above the horizon before it rises.

Explain the laws of Refraction?

The laws of refraction are as follows:

1. The ration between the sine of the angle of incidence (in medium 1) and sine of the angle of refraction (in medium 2) is constant. This constant is called the refractive index of medium 2 with respect to 1. This is known as Snell’s law.

₁µ_{2  = sin i / sinr}

2. The incident ray, the refracted ray and the normal to the point of incidence all lay in the same plane.

Which beams of light rays form a real image and virtual image?

A Convergent beam of light rays forms a real image whereas a Divergent beam of light rays from a virtual image.

In which direction does light bend due to refraction?

If light travels from a medium of lower density to a higher density medium, it bends towards the normal. Whereas, if it travels from a higher density medium to a lower density medium, it bends away from the normal.

How many images are formed by inclined mirrors?

If θ  is the angle between the plain mirrors, then the number of images formed is given by the formula:

n = (360 / θ) - 1

Definitions: 

1. Light: It is a form of energy which is emitted by a luminous body and causes a sensation of sight when incident on the eye.
2. Luminous bodies: Bodies which emit light and act as sources of light are called luminous bodies.
3. Non-luminous bodies: Bodies which do not emit light but are made visible when light falls on them are called non-luminous bodies.
4. Medium: Substance through which light passes is called a medium
5. Transparent substances: A substance through which light can travel freely is called transparent substance or medium.
6. Translucent substance: A substance which partially allows light ways to pass through it is called a translucent substance.
7. Opaque substance: A substance that doesn’t allow light to pass through it is called an opaque substance.