# PHY3: LIGHT

##### This unit is the introduction of light and has sub topics like rays and beams, shadows and the eclipse.

LIGHT [OPTIONS]

Light is a form of energy.  Light consists of streams of tiny wave like packets of energy called photons. Which travel at a speed of 3 x 108 meters per second?

The main source of light is the sun. Other source of light includes; – electric lamps, glowworms, torches etc. light can be transformed or changed to other forms of energy and vice versa.

Objects that provide their own light include those above, are called luminous objects.

We only see light enters our eyes. Most objects do not produce light. They are called non luminous objects. Such objects can only be seen if they reflect that falls on their own eyes. These include; – the moon, pen, people and objects that we see around us.

RECTILINEAR PROPAGATION OF LIGHT

An experiment to investigate how light travels

Apparatus

Source of light

3 pieces of cardboard with a small hole in the centre of earth support/plasticize

Procedure

Put the string through the holes in the cards and pull it tight. Uses palatine fix the cards in their positions. Remove the shrink from the other end A. displace the centre card a little and look through the holes again.

Observations

When the middle card is in place light can be seen all the holes when distanced.

Conclusion

A tight strong is a straight line. When the 3 holes are in the straight line, light can be seen at A. when the middle is placed off a little no light is seen at A. this shows that light coming from the source to the eye moves in a straight line.

THIS VIDEO GIVES A BRIEF INTRODUCTION OF LIGHT

RAYS AND BEAMS

A ray

It refers to a narrow stream of light energy that is coming through a small hole in the screen. A ray is the path direction a path of light takes. It represented by a line with an arrow.

A beam

This is a stream of light energy and is represented by a number of rays. A beam can be parallel, divergent or convergent.

• parallel beams

• Divergent beams

• Convergent beams

The pin-hole camera consists of a blackened box with a small hole at 1 one and a screen of trashing paper/frost glass at the other end. An image is seen on the screen and it is upside down. This is due to light passing from the object through the pin to the screen.

A blurred image [unclear image] when the hole is large. This is so because of multiple production of images from these small holes that are close together causing over lapping images.

However, the object will be brighter if the hole is large. Magnification is the ratio of object [size] to the image height or the ratio of image distance from the pinhole to object distance from the pinhole.

Those for a bigger image the distance of the object from the pinhole must be small compared to the distance between the pin hole and screen.

A VIDEO ABOUT RAYS AND BEAMS

These are formed when the path of light is blocked by an obstacle. The edges of shadows are usually sharp indicating that light travels in straight lines.

There are two types of shadows;-

1. Umbra
2. Penumbra

Umbra is total darkness and penumbra is partial darkness.

A point source produces total darkness only

[Umbra and penumbra]

An extended hole /source give rise to both partial complete shadows.

ECLIPSE

An ellipse occurs when the path of light from the sun is blocked by a planet.

Solar eclipse

This is an eclipse of the son. It occurs when the moon passes between the earth and sun and all three are in the straight line.

How umbra and penumbra are formed on the earth.

B – Experience total eclipse

A – Experiences partial eclipse

C – Experiences no eclipse

Annular eclipse

B – Partial eclipse

A – Sun appears as a ring of light

Annular eclipse occurs when the distance of the moon from the earth is such that the trip of the umbra fails to reach the earth’s surface

Lunar eclipse

This eclipse occurs when the earth passes between the moon and the sun hence the eclipse is experienced by the moon.

THIS VIDEO SHOWS THE DIFFERENCE BETWEEN THE LUNAR AND SOLAR ECLIPSE.

YOU