Optics (GA Physics) Notes for SSC CGL, CHSL & Railways

# Optics (Ray Optics, Wave Optics)

Optics is the branch of physics which involves the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it.

• Optics usually describes the behaviour of visible, ultraviolet, and infrared light.
• Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum.
• Speed of Light: 3 x 108 ms-1
• Time for light to reach on Earth from Sun: 8 minute 20 seconds
• When light falls on the surface of an object it can either be Absorbed, Transmitted or Reflected.

## 1. Ray Optics (Geometrical optics)

It describes light propagation in terms of rays.

• The ray in geometric optics is an abstraction useful for approximating the paths along which light propagates under certain circumstances.

### 1.1 Reflection of Light

The process through which light rays falling on the surface on an object are sent back (reflected) is called as reflection of light.

#### 1.1.1 Law of Reflection of Light

The law of reflection states that the incident ray, the reflected ray, and the normal to the surface of the mirror all lie in the same plane.

• The angle of incidence is equal to the angle of reflection. ### 1.2 Refraction of light

Refraction is the change in direction of wave propagation due to a change in its transmission medium.

• The cause of refraction is due to the different light speed in different media.
• When a ray of light enters from one medium to another medium, its frequency and phase do not change, but wavelength and velocity changes.
• The stars appear to twinkle due to refraction.

#### 1.2.2 Effects of Refraction

• When light travels through a denser medium towards a rarer medium it deviates away from the normal, therefore a pond appears shallower.
• A coin appears at lesser depth in water
• Writing on a paper appears lifted when a glass slab is placed over the paper.
• The Sun is visible to us before actual sunrise and after actual sunset.

#### 1.2.3 Critical Angle

The angle of incidence in a denser medium for which the angle of refraction in rarer medium becomes 90°, is called the critical angle.

#### 1.2.4 Total Internal Reflection

TIR is the phenomenon which occurs when a propagated wave strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface.

##### Conditions for TIR
1. Light should be incident from denser to rarer medium
2. The angle of incidence should be greater than the critical angle.
##### Examples of TIR:
• Sparkling of diamond, mirage and looming, shining of air bubble in water or glass, optical fibre.

### 1.3 Mirrors

#### 1.3.1 Plane Mirror

• Images formed are laterally inverted, virtual and erect
• The length of a plane mirror in which a person can see his/her full image is half the height of the person.
• Used in looking glasses, solar cooker, barber’s shop, etc.
• If the object moves with a velocity ‘v’ towards or away from the mirror, its image appears to move with a velocity ‘2v’.

#### 1.3.2 Spherical Mirrors (Concave, Convex)

1. Concave Mirrors (Forms real and inverted images), Uses: Shaving Mirror, Headlights of vehicles, searchlight, opthalmoscope, solar cookers
2. Convex Mirrors (Forms virtual, erect and diminished images), Uses: Sodium Reflactor Lamp

Important points related to the Spherical Mirrors

• Centre of Curvature (c): The centre of the hollow glass sphere of which the mirror is a part.
• The radius of Curvature (R): The radius of hollow sphere of which the mirror is a part.
• Pole (P): The mid point of a spherical mirror is called pole.
• Focus: When a parallel beam of light rays is incident on a spherical mirror then after reflection it meets or appears to meet at a point on the principal axis, called focus of the spherical mirror.
• Focal length (f): f = R / 2

### 1.4 Lenses (Concave, Convex)

A lens is a uniform refractive medium bounded by two spherical surface or one plane.

#### 1.4.1 Power of lenses (P)

```   P = 1 / f
```
• SI unit: Dioptre (D)
• A biconcave or biconvex lens has two centre’s of curvature and two foci.

#### 1.4.2 Concave Lens (Convergent Lens)

• At least one inward curve
• real or virtual images

#### 1.4.3 Some Facts Regarding Lenses

• If a convex and a concave lens of the same focal length are put in contact the combination acts as a glass slab.
• If a convex and a concave lens of the same focal length are placed co-axially with a small separation between them, the combination acts as a convex lens.
• If a lens is cut into two equal halves with a plane perpendicular to the principal axis, the focal length of each part doubles.
• If a lens is cut into two equal halves with a plane along the principal axis, the focal length of each part remains unchanged.
• An air bubble in water behaves as a concave lens.
• When a lens is placed in a medium of higher density (higher refractive index), the nature of the lens reverses i.e. a converging lens behaves as a diverging lens & vice-versa.
• When a lens is placed in a medium of equal density, it acts as a glass slab. Its focal length becomes infinity and power becomes zero.

### 1.5 Prism

Prism is a uniform transparent refracting medium bounded by plane surfaces inclined at some angles forming a triangular shape. • When a white light is incident on a glass prism, it disperses into its seven colour components in the sequence of VIBGYOR, and this is known as the dispersion of white light.
• The refractive index of glass is maximum for violet colour and minimum for the red colour of light, therefore the violet colour of light deviated maximum and red colour of light deviated least.

### 1.6 Common Defects of Vision

#### 1.6.1 farsightedness (Hypermetropia)

A person suffering from this defect can see objects far away but cannot see the nearer object.

• Eye forms the image of an object behind the retina.
• The defect can be corrected by using a convex lens of suitable focal length.

#### 1.6.2 shortsightedness (Myopia)

A person suffering from this defect can see the nearer objects clearly but is unable to see objects far away.

• In this defect, the image is formed in front of retina.
• This defect is corrected by using a concave lens of suitable focal length.

## 2. Wave Optics (Physical optics)

The branch of optics that studies interference, diffraction, polarization, and other phenomenons.