Refraction of Light

Chapter 18, Section 1

Refraction

When light encounters a transparent or translucent medium, some light is reflected from the surface of the medium and some is transmitted through the medium.

When light crosses a boundary between two mediums, it bends. A phenomenon called refraction.

The bending of light, called refraction, was first studied by René Descartes and Willebrord Snell around the time of Kepler and Galileo (early 1600s).

Refraction Refraction is the bending of light as it travels

from one medium to another. The bending occurs because of a difference in

the index of refraction of the mediums. Refraction occurs when the velocity of light

changes.

Angle of Refraction

The angle between the refracted ray and the normal is called the angle of refraction

Angle of refraction

Refraction

Look at the figure above: Identical rays of light start in air and pass into three different mediums at the same angle.

When light travels from air to glass it moves from material having a lower index of refraction to one with a higher index. In this case, the light is bent toward from the normal.

Angle of Refraction When a light ray goes from a gas to a

liquid or a solid, the light ray slows down and the angle of refraction is bent towards the normal

When a light ray goes from a liquid or a solid to a gas, the light ray speeds up and the angle of refraction is bent away from the normal

Dispersion of Light Dispersion is the separation of white light into a

spectrum of colors. A prism is not the only means of dispersion of light.

A rainbow is a spectrum formed when sunlight is dispersed by water droplets in the atmosphere. Sunlight that falls on a water droplet and is

refracted. Since each color has a difference wavelength, it is refracted at a slightly different angle.

Sometimes a second rainbow is observed, this occurs when light rays are reflected twice inside a water droplet.

Examples of Refraction

Total Internal Reflection If the angle of incidence is great enough, the

angle of refraction will be so large, that light will not exit the medium.

Instead, it will be reflected. This is known as total internal reflection.

The minimum angle of incidence that will cause this is known as the critical angle

Total Internal Reflection

Total internal reflection causes some curious effects.

Suppose that you are looking up at the surface from underwater in a calm pool.

You might see an upside-down reflection of a nearby underwater object or a reflection of the bottom of the pool itself.

The surface of the water acts like a mirror.

Total Internal Reflection

Likewise, when you are standing on the side of a pool, it is possible for things below the surface of the water to not be visible to you.

When a swimmer is underwater, near the surface, and on the opposite side of the pool from you, you might not see him or her.

This is because the light from his or her body does not transmit from the water into the air, but is reflected.

Fiber OpticsAn Application of Total Internal Reflection

Optical fibers are an important technical application of total internal reflection. The light traveling through the transparent fiber

always hits the internal boundary of the optical fiber at an angle greater than the critical angle, so all of the light is reflected and none of the light is transmitted through the boundary.

Thus, the light maintains its intensity over the distance of the fiber.

On a hot summer day, as you drive down a road, you see what appears to be the reflection of an oncoming car in a pool of water. The pool, however, disappears as you approach it.

The mirage is the result of the Sun heating the road. The hot road heats the air above it and produces a thermal layering of air that causes light traveling toward the road to gradually bend upward.

This makes the light appear to be coming from a reflection in a pool.

Mirages

Convex and Concave Lenses

Chapter 18, Section 2

Lenses A Lens is a piece of transparent material,

such as glass or plastic, which is used to focus light and form an image.

A convex lens is thicker in the center A concave lens is thicker on the outside An achromatic lens is a system of two or

more lenses, such as a convex lens with a concave lens, that have different indices of refraction

Lenses

Lenses are found in telescopes, microscopes, magnifying glasses, eyeglasses, and the human eye

Focal Point The focal point of a lens is the point in

space where parallel light rays meet after passing through the lens.

Symbol for the focal point is f

Convex Lenses When rays of light pass through a Convex lens

(thicker in the middle), they are bent inwards. Focal point is on the opposite side of the lens as

the incoming light rays. A Convex lens forms a real image that is inverted

and larger than your object. Often referred to as a converging lens

Concave Lenses When rays of light pass through a

Concave lens (thicker on the outside), they are bent outwards.

Focal point is on the same side of the lens as the incoming light rays.

Concave lens forms a virtual image Often referred to as a diverging lens

Drawing Ray Diagrams

For a Convex Lens

Ray Diagram - Converging Lens (Convex Lens)

01/26/2011 IB Physics HL 221

Object

Image

ff

Drawing Ray Diagrams - Convex Lens

A ray diagram is the best way to understand what type of image is formed by a lens.

There are three rays that we draw that follow the rules of how light rays are bent by the lens:

1. The first ray passes through the center of the lens is not deflected at all.

2. The second ray travels parallel to the axis and passes through the far focal point.

3. The third ray passes through the near focal point emerges parallel to the axis.

The image is formed where all three rays converge. It is inverted

Thin Lens Model The thin lens model is an assumption is

made that all refractions occurs at a plane, called the principal plane that passes through the center of the lens.

All lenses we will discuss in this chapter use the thin lens model.

The thin lens equation relates the focal length of a spherical thin lens, the object position, and the image position

Variables for Thin Lens Equation focal length, f

Distance between the focal point and the lens

image distance, di Distance image is from the lens

object distance, do Distance object is from the lens

Thin-lens equation

Where:

f – focal length

di – image distance

do – object distance

oi ddf

111

Practice # 1 When an object is placed 0.03 m in front of a converging

lens, a real image is formed 0.06 m in back of the lens. Find the focal length of the lens

Given:• do = 0.03 m• di = 0.06 m

Unknown:• f = ?

Using the thin lens equation

Ans: 0.02 m50

1

501

03.0

1

0.06

11

111

f

f

f

ddf oi

Practice # 2 An object is placed 0.25 m in front of a converging lens

of focal length 0.09 m. Find the image distance. Given:

• do = • f =

Unknown:• di = ?

Practice # 3 An object placed 0.30 m from a converging lens. The

focal length of the lens is 0.11 m. Calculate the image distance

Given:• do = • f =

Unknown:• di =

Applications of Lenses

Chapter 18, Section 3

Applications of Lenses

People see objects that they could not otherwise see by using lenses