ppt on refraction and lenses by pg

Refraction and Lenses(Physics)

Pritam Ghanghas

K. Rohit

Ashok Bishnoi

Refraction Refraction is based on the idea that LIGHT is passing through one

MEDIUM into another it will bend towards or away the normal and this phenomenon is called refraction of light. The question is, WHAT HAPPENS?

Suppose you are running on the beach with a certain velocity when you suddenly need to run into the water. What happens to your velocity?

IT CHANGES!

Refraction Fact #1: As light goes from one medium to another, the velocity CHANGES!

RefractionSuppose light comes from air, which in this case will be considered to

be a vacuum, strikes a boundary at some angle of incidence measured from a normal line ,and goes into water.

The ratio of the two speeds can be compared.

The denominator in this case will ALWAYS be smaller and produce a unitless value greater or equal to 1. This value is called the new medium’s INDEX OF REFRACTION, n.

All substances have an index of refraction and can be used to identify the material.

RefractionSuppose you decide to go spear fishing, but unfortunately you

aren’t having much luck catching any fish.

The cause of this is due to the fact that light BENDS when it reaches a new medium. The object is NOT directly in a straight line path, but rather it’s image appears that way. The actual object is on either side of the image you are viewing.

Refraction Fact #2: As light goes from one medium to another, the path CHANGES!

Refraction is when light bends as it passes from one medium into another.

When light traveling through air passes into the glass block it is refracted towards the normal.

When light passes back out of the glass into the air, it is refracted away from the normal.

Since light refracts when it changes mediums it can be aimed. Lenses are shaped so light is aimed at a focal point.

normal

normal

air

air

θr

θi

θr

θi

glass block

The first telescope, designed and built by Galileo, used lenses to focus light from faraway objects, into Galileo’s eye. His telescope consisted of a concave lens and a convex lens.

Light rays are always refracted (bent) towards the thickest part of the lens.

convex lens

concave lens

light from object

LensesThere are 2 basic types of lenses

A converging lens (Convex) takes light rays and bring them to a point.

A diverging lens (concave) takes light rays and spreads them outward.

optical axis

•F

Light rays that come in parallel to the optical axis still diverge from the focal point.

The first ray comes in parallel to the optical axis and refracts from the focal point.

optical axis

•F

optical axis

•F

The first ray comes in parallel to the optical axis and refracts from the focal point.

The second ray goes straight through the center of the lens.

optical axis

•F

The first ray comes in parallel to the optical axis and refracts from the focal point.

The second ray goes straight through the center of the lens.

The light rays don’t converge, but the sight lines do.

optical axis

•F

The first ray comes in parallel to the optical axis and refracts from the focal point.

The second ray goes straight through the center of the lens.

The light rays don’t converge, but the sight lines do.

A virtual image forms where the sight lines converge.

Convex (Converging) LensMuch like a mirror, lenses also take light rays from infinity and converge

them to a specific point also called the FOCAL POINT, f. The difference, however, is that a lens does not have a center of curvature, C, but rather has a focal point on EACH side of the lens.

optical axis

•F

•2F

•2F

•O

•F

When we put object between O and F,

The image properties will be: -Virtual errect

Big in size

Behind the mirror

optical axis

•F

•2F

•2F

•F

When we put object at F,

The image properties will be: -

Real inverted

Very large in size

Meet at infinity

optical axis

•2F

•F

•F

•2F

When we put object between 2F and F,

The image properties will be: -

Real inverted image

Big in size

Beyond 2F

optical axis

•F

•2F

•F

•2F

When we put object at 2F,

The image properties will be: -

Real inverted

Same in size

Meet at 2F

optical axis

•2F

•F

•F

•2F

When we put object beyond 2F,

The image properties will be: -

Real inverted

Small in size

Meet between F and 2F

•2F

•F

•F

•2F

optical axis

When we object from infinity,

The image properties will be: -

Point size at F

Meet at F

Real inverted

optical axis

•F

•2F

When we put object between F and 2F

The image properties will be: -

Virtual erect image

Between 2F and O.

Small in size

optical axis

•F

•2F

When we object rays coming from infinity

The image properties will be: -

Virtual erect image

At F.

f = focal length

u = object distance

v = image distance

An equation which represents the relation between focal lengh (f) of lens,

object distance (u) and image distance (v) is called lens formula.