1. The value of  sini/sinr is called the refractive index of the medium and it gives you an indication of its light-bending ability.
   
2. In SPM, when we say “refractive index”, what we mean is the absolute refractive index of a substance. The absolute refractive index of a substance is the refractive index where light ray travels from vacuum (or air) into the substance.

Refractive Index and the Speed of light

or

( Note that the greater the refractive index of a medium, the lower is the speed of light. The more light is slowed, the more it is bent. )

 

1. The incident and refracted rays are on opposite sides of the normal at the point of incidence, and all three lie in the same plane.
2. The value of sini/sinr is constant for light passing from one given medium into another. This is known as Snell's law.

 

1. Light rays are bent when they pass at an angle in or out of materials such as glass and water. The effect is called refraction.
2. Light passing into an optically denser medium is bent towards the normal; light passing into an optically less dense medium is bent away from the normal.
3. Materials such as glass, water and paraffin are said to be optically denser than air.

 

Finding the Position and Size of the Image

1. Any two rays are sufficient to fix the position and size of the image. Look for the point where the rays cross after reflection from the mirror.
2. The interception of the two rays is the focus of the ray.

Example

The Ray Diagram and the Types of Image

The nature and size of the image formed by concave mirrors depends on the distance of the object from the mirror. This will be explained in the following sections.

 

Rules in Drawing Ray Diagram

Rule No. 1
The ray of light through C will be reflected back through C.

Rule No. 2
The ray of light parallel to the principal axis will be reflected through F.

Rule No. 3

The ray of light through F will be reflected parallel to the principal axis.

 

Curved Mirror

1. A curve is part of a circle. Therefore
   1. the centre of the circle will also be the centre of the curve and is called the centre of curvature, and
   2. the radius of the circle will be equal to the radius of the curve, called the curvature radius.

Important Terms

All rays parallel to the principle axis will focus at F

 

Steps to draw a ray diagram for an image in a plane mirror 

Step 1

( Draw the virtual image. Distance of object = Distance of image )

Step 2

( Draw 2 reflected rays, one from the image to the top of the eye and the other one from the image from the bottom of the eye. )

Step 3

( Draw the respective incident rays for the reflected rays you draw in step 2. )

 

 

Plane Mirror

Images in plane mirrors

(Reflection of light on a plane mirror)

1. Figure to the right shows how, by reflecting light, a plane mirror forms an image of a point source of light such as a small light bulb.
2. The image forms in a mirror is
   1. Upright
   2. Virtual
   3. Laterally inverted
   4. Same size as the object
      

 

Note: Both the angle of incident and angle of reflection must be measured from the normal.

Laws of Reflection

1. The law of reflection state that
   1. The angle of incidence is equal to the angle of reflection; the ray leaves the surface at the same angle as it arrives.
   2. The incident ray, the reflected ray and the normal all lie in the same plane; all three could be drawn on the same flat piece of paper
      

Type of Mirror