Class 10 Science (physics) Light : Reflection and Refraction

 ~ Magnification

Magnification produced by a spherical mirror gives the relative extent to which the image of an object is magnified with respect to the object size. It is expressed as the ratio of the height of the image to the height of the object. It is usually represented by the letter m.

If h is the height of the object and h ' is the height of the image, then the Magnification m produced by a spherical mirror is given by 

The magnification m is also related to the object distance (u) . It can be expressed as : 

You may note that the height of the object is taken to be positive as the object is usually placed above the principal axis. The height of the image should be taken as positive for virtual images. However, it is to be taken as negative for real images. A negative sign in the value of the magnification indicates that the image is real. A positive sign in the value of the magnification indicates that the image is virtual. 

 Let's do A Numerical. 

• A convex mirror used for rear-view on an automobile has a radius of curvature of 3.00 m. If a bus is located at 5:00 m from this mirror, find the position,nature and size of the image. 

Solution=  

Refraction of Light 

Light seems to travel along straight-line

 paths in a transparent medium. 

What happens when light enters from

one transparent medium to another? 

Does it still move along a straight-line

 

path or change its direction? We shall

 

recall some of our day-to-day

 experiences. 

You might have observed that the bottom

of a tank or a pond containing water

appears to be raised. Similarly, when a

thick glass slab is placed over some

printed matter, the letters appear raised

when viewed through the glass slab.Why

does it happen? Have you seen a pencil

partly immersed in water in a glass

tumbler?  It appears to be displaced at the

interface of air and water. You might have

observed that a lemon kept in water in a

glass tumbler appears to be bigger than

its actual size, when viewed from the

sides. How can you account for such

experiences? 

Let us consider the case of the apparent

displacement of a pencil,partly immersed

in water. The light reaching you from the

portion of the pencil inside water seems to

come from a different direction, compared to the 

Part above water. This makes the pencil 

appear to be displaced at the interface. 

For similar reasons, the letters appear to be 

raised, when seen through a glass slab 

placed over it. 

Does a pencil appear to be displaced 

to the same extent, if instead of water,

we use liquids like kerosene or

turpentine ? Will the letters appear to 

rise to the same height if we replace a 

glass slab with a transparent plastic 

slab? You will find that the extent of 

the effect is different for differ pair of 

media. 

In fig 10.10, EO is the incident ray, OO' is the

refracted ray and O' H is the emergent 

ray . 

You may observe that the emergent ray

is parallel to the direction of the

incident ray. Why does it happen so ? 

The extent of bending of the ray of  

light at the opposite parallel faces AB 

(air-glass interface) an CD (glass-air

 interface) of the rectangular glass slab

 is equal and opposite. This is why the

ray emerges parallel to the incident 

ray. However, the light ray is shifted 

sideward slightly. 

Now you are familiar with the refraction of

light. Refraction is due to change in the 

speed of light as it enters from one 

transparent medium to another. 

Experiments show that refraction of 

light occurs according to certain laws. 

The following are the laws of reflection of light. 

i) the incident ray, the refracted Ray and the normal to the interface of two transparent in media at the front of incidence, all lie in the same plane.

ii) the ratio of sine of angle of incident to the Sine of angle of refraction is a constant, for the light of a given colour and for the given pair of media. This law is also known as snell's  law of refraction.

If i is the angle of incidence and r  is the angle of refraction, then, 

This constant value is called the refractive 

index of the second medium with respect 

to the first. Let us study about refractive index

in some detail.

   TO Be Continued.....