Revision: Light Energy Physics ICSE ICSE Class 8 CISCE

Materials that are not able to allow light to pass through, are called opaque material.

The refractive index of a medium is defined as the ratio of the speed of light in vacuum (or air) to the speed of light in that medium.

Materials that allow light to pass through completely are known as transparent materials.

Refracted light is the part of light enters into the other medium and travels in a straight path but in a direction different from its initial direction and is called the refracted light.

1. Incident ray: The ray of light which falls on the mirror surface is called the incident ray.
2. Point of incidence: The point at which the incident ray falls on the mirror is called the point of incidence.
3. Normal: The normal is a line at right angles to the mirror surface at the point of incidence.
4. Reflected ray: The ray of light which is sent back by the mirror is called the reflected ray.
5. Angle of incidence: The angle of incidence is the angle made by the incident ray with the normal at the point of incidence.
6. Angle of reflection: The angle of reflection is the angle made by the reflected ray with the normal at the point of incidence.

The ray that enters the second medium after crossing the boundary is called the refracted ray.

A normal is an imaginary line drawn perpendicular to the boundary at the point of incidence.

When travelling obliquely from one medium to another, the direction of propagation of light in the second medium changes. This phenomenon is known as refraction of light.

OR

Light changes its direction when going from one transparent medium to another transparent medium. This is called the refraction of light.

OR

The bending of the light ray from its path in passing from one medium to the other medium is called 'refraction' of light.

OR

When a ray of light impinges on a polished, smooth, shiny surface, the rebounding of light within the same medium is called reflection of light.

The change in the direction of the path of light when it passes from one transparent medium to another transparent medium is called refraction. The refraction of light is essentially a surface phenomenon.

or

When light passes from one transparent medium to another, its speed and direction change. This is called refraction.

Light rays that are parallel to the principal axis of a concave mirror converge at a specific point on its principal axis after reflecting from the mirror. This point is known as the principal focus of the concave mirror.

The perpendicular distance XY between the path of the emergent ray BC and the direction of the incident ray OD is called the lateral displacement.

It is defined as the ratio of the velocity of light in medium 1 to the velocity of light in medium 2.

A prism is a transparent medium bounded by five plane surfaces with a triangular cross-section.

On passing white light through a prism, the band of colours seen on a screen is called the spectrum.

or

The band of the coloured components of a light beam is called its spectrum.

The phenomenon of the splitting of white light by a prism into its constituent colours is known as dispersion of light.

When a beam of white light or composite light is refracted through any transparent media such as glass or water, it is split into its component colours. This phenomenon is called ‘dispersion of light’.

The phenomenon of splitting of white light by a prism into its constituent colours is known as dispersion.

OR

The splitting of light into its component colours is called dispersion.

OR

The process of separation of light into its component colours while passing through a medium is called the dispersion of light.

OR

The phenomenon in which white light splits into its constituent colours when it passes through a prism or another medium is called dispersion of light.

The angular separation between the two extreme rays of a dispersed beam of light is called angular dispersion.

The focus of a concave mirror is a point on the principal axis of the mirror, where all the rays travelling parallel to the principal axis and close to it after reflection from the mirror converge to that point.

The linear distance between the pole and the center of curvature is called the radius of curvature.

It is a point on the principal axis, where a beam of light, parallel to the principal axis, after reflection actually meet.

An imaginary line passing through the pole and the centre of curvature of a spherical mirror is called principal axis.

The centre of a hollow sphere of which the mirror forms a part is called the centre of curvature.

Aperture is the distance between the extreme points on the periphery of the mirror.

The linear distance between the pole and the principal focus is called focal length.

Normal to the surface of a mirror at any point is the straight line at the right angle to the tangent drawn at that point.

Pole “is the mid-point of the mirror”.

“A mirror made by silvering the outer or the bulging surface such that the reflection takes place from the concave surface.” Centre of curvature is towards the reflecting surface.

“A mirror made by silvering the inner surface such that reflection takes place from the bulging surface” is called Convex Mirror.
The Centre of curvature is towards the silvered surface.

“A mirror which is made from a part of a hollow sphere is called Spherical Mirror.

Principal focus of a spherical mirror is a point on the principal axis of the mirror, where all the rays travelling parallel to the principal axis and close to it after reflection from the mirror, converge to or appear to diverge from.

Pole is the centre of the reflecting surface, in this case, a spherical mirror.

The distance between the pole and the principal focus is called the focal length (f) of a spherical mirror.

 Centre of curvature is the centre of the imaginary sphere to which the mirror belongs.

Mirrors whose reflecting surfaces are spherical are called spherical mirrors.

OR

A spherical mirror is a part of a hollow sphere, whose one side is silvered and coated with red oxide and the other side is the reflecting surface.

OR

A spherical mirror is a piece cut out of a spherical surface, which can be concave or convex.

A spherical mirror, whose reflecting surface is curved inwards, that is, faces towards the centre of the sphere, is called a concave mirror.

OR

A concave mirror is one whose reflecting surface is towards the centre of the sphere of which the mirror is a part.

OR

The reflecting surface is on the inner side of the sphere (converging mirror).

A spherical mirror whose reflecting surface is curved outwards, is called a convex mirror.

OR

A convex mirror is one whose reflecting surface is away from the centre of the sphere of which the mirror is a part.

OR

The reflecting surface is on the outer side of the sphere (diverging mirror).

The radius of the sphere of which the reflecting surface of a spherical mirror forms a part is called the radius of curvature of the mirror. It is represented by the letter R.

OR

The radius of the sphere of which the mirror forms a part, is called the 'radius of curvature' of the mirror.

A straight line passing through the pole and the centre of curvature of a spherical mirror. This line is called the principal axis.

OR

The straight line joining the pole and the centre of curvature of the mirror and extended on both sides is called the 'principal axis' of the mirror.

The centre of the reflecting surface of a spherical mirror is a point called the pole. The pole is usually represented by the letter P.

OR

The central point of the reflecting surface of the mirror is called the 'pole' of the mirror.

The reflecting surface of a spherical mirror forms a part of a sphere. This sphere has a centre. This point is called the centre of curvature of the spherical mirror. It is represented by the letter C.

OR

The centre of the sphere of which the mirror forms a part, is called the ‘centre of curvature' of the mirror.

INCIDENT RAY AB: The ray light AB which is in air strikes the glass slab at B.
Or
“A ray of light falling on the surface separating the two media.”
REFRACTED RAY BK: A ray of light which after passing the first medium is in second medium i.e. ray BK.
“A ray of light travelling in other medium in the changed direction.”
ANGLE OF INCIDENCE: “The angle which the incident ray makes with the normal is called angle of incidence.”
i.e. ∠i
ANGLE OF REFRACTION: “The angle which the refracted ray makes with the normal is called angle of refraction.”
i.e. ∠r

Principal focus (F): The point on the principal axis of the spherical mirror where the rays of light parallel to the principal axis meet or appear to meet after reflection from the spherical mirror.

m = `h_i/h_o`

The laws of refraction are fundamental for board examinations and objective tests.

First law

The incident ray, the refracted ray, and the normal at the point of incidence all lie in the same plane.

Second law

For a given pair of media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction remains constant.

\[\frac {\text {sin i}}{\text {sin r}}\] = constant

This constant is called the refractive index of the second medium with respect to the first medium.

Ray 2 shows partially reflected ray.

• Refractive index (µ) = c / V, where c is the speed of light in vacuum and V is the speed in the medium.
• The refractive index of a medium is always > 1 because the speed of light in any medium is less than in a vacuum.
• If µ₁ = µ₂ or the angle of incidence = 0°, the ray of light passes undeviated.
• Wavelength in medium A′ = A / µ; wavelength decreases in denser medium and increases in rarer medium.
• Refractive index decreases with decreasing speed of light, and is maximum for violet light and minimum for red light.

• Dispersion is the splitting of white light into seven colours (VIBGYOR) when it passes through a prism or similar transparent medium.
• Human eyes can detect light with wavelengths ranging from 400 nm (violet) to 700 nm (red).
• Different colours travel at different speeds in a medium like glass, so each colour has a different refractive index.
• Violet light bends the most, and red light bends the least, as it passes through a prism, producing a spectrum.
• A rainbow is formed due to dispersion, refraction, and internal reflection of sunlight by raindrops acting as tiny prisms.