Revision: Light Science SSLC (English Medium) Class 9 Tamil Nadu Board of Secondary Education

Light is a form of Energy i. e. The external physical cause that affects our eye to produce the sensation of vision.

The phenomenon due to which a parallel beam of light traveling through a certain medium, on striking some polished surface, bounces off from it, as a parallel beam, in some other direction, is called regular reflection.

The principal axis is the straight line passing through the pole and the centre of curvature.

The bouncing of light by any smooth or polished surface is called.

The phenomenon of bouncing back of light rays in the same medium on striking a surface is called reflection of light.

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

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

“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 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.

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

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

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

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

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

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.

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.

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

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

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

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

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

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.

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.

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.

The distance of the principal focus from the pole is called the focal length (f).

In a spherical mirror, the distance of the object from its pole is called the object distance (u).

The distance of the image from the pole of the mirror is called the image distance (v).

The ratio of the height of an image (h') to the height of an object (h) is known as linear magnification 

That is, 

`mh/h` 

where, h^' = height of image
            h =  height of object 

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.

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.

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 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.

The phenomenon where light rays are completely reflected back into a medium instead of being refracted into another medium is called total internal reflection.

or

Complete reflection of a ray of light at the interface of an optically denser medium and a rarer medium, back into the denser medium.

When a ray of light propagates from a denser medium to a rarer medium, the angle of incidence for which the angle of refraction is 90° is called the critical angle.

The angle of incidence in the denser medium corresponding to an angle of refraction of 90° in the rarer medium is called the critical angle.

n = \[\frac {360°}{θ}\]

• If n is even → N = n − 1
• If n is odd → N = n (object not on bisector); N = n − 1 (object on bisector)
• If n is a fraction → N = integral part of n

\[\frac {1}{v}\] + \[\frac {1}{u}\] = \[\frac {1}{f}\]

Magnification (m) = \[\frac{\text{Height of the image (}h'\text{)}}{\text{Height of the object (}h\text{)}}\] = \[\frac {h'}{h}\]

Magnification in terms of object and image distances:

Magnification (m) = \[\frac {h'}{h}\] = -\[\frac {v}{u}\]

• The angle of incidence ∠i = angle of reflection ∠r.
• The incident ray, reflected ray, and normal lie in one plane; both rays are on either side of the normal.

1. The angle of reflection is equal to the angle of incidence.
2. The incident ray, the reflected ray, and the normal lie in the same plane.
3. The incident ray and the reflected ray are on opposite sides of the normal.

• Reflection occurs when light bounces off a smooth surface like a mirror, following fixed laws.
• Plane mirrors always form virtual, erect, and same-sized images that are laterally inverted.
• Curved surfaces (like a spoon) act as spherical mirrors, changing the image size and orientation depending on the object's position.

• Pole (mirror) or optical centre (lens) is the origin; principal axis is the X-axis.
• Distances to the right are positive, to the left are negative; heights above the axis are positive, below are negative.
• Concave mirror: $f$ and R are negative; Convex mirror: $f$ and R are positive.
• Real images: image distance and magnification are negative; Virtual images: both are positive.
• Lenses are always negative; they are positive for real images and negative for virtual images; they are positive for convex lenses and negative for concave lenses.

• 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.