Light

Light is a form of energy transmitted as electromagnetic waves and can be detected by the human eye.

About 2000 waves of light would fit across a millimetre; Light can be changed to other forms of energy

a) Light to Electrical Energy e.g. solar cell

b) Light to Chemical e.g. photosynthesis

c) Light to Kinetic e.g. Crooke’s Radiometer, spins in direct light.

d) Light to Heat e.g. black bulb thermometer, mercury rises when bulb in direct light..

The Applications of Light in Our Lives

a) Vision, Detect the size, shape, motion and form of things. 70% of the sensory information comes from eyes.

b) Light is the source of all food – light energy is converted to the chemical energy in food by photosynthesis.

c) Light can be used to produce electricity – solar cells, solar panels.

d) Light transmitted along optic fibres is used in communication and information transfer.

Sources of Light

a) Light Producing or Luminous Object

A luminous object produces and emits light. The Sun, flames and lit light bulbs are luminous. An object gives off light if its surface temperature is greater than 600°C. A bulb operates at 2,500°C but light is only a few percent of the emitted energy.

b) Light Reflecting Object Non-luminous objects do not emit light – surface temperature is less than 600°C. Non-luminous object can only be seen if light shines on them and all or some of that light bounces off them into our eye. Non-luminous objects can only be seen due to reflected light. e.g. book, desk, tree, you, me, mirror.

Transmission of Light is the transfer of light from one location to another. Light travels in a straight line extremely fast. Light can pass through a vacuum. Light can also pass through various types of solids, liquids and gases. Materials that light can pass through are described as translucent or transparent. Translucent means that the light is deflected so objects cannot be seen clearly through them e.g. frosted bathroom glass. Transparent means that the light passes through without much change such that objects can be seen clearly through them. Light can pass through glass, water and air (gases) Materials that block light are described as opaque.

Speed of Light is about 300 million metres a second Light travels a million km in 3.33 seconds. In 3.33 seconds sound would only travel 1.13 km. The Earth is 150 million km from the Sun it takes light 8 minutes 20 seconds to reach the Earth. Therefore we see the Sun in its past – just over 8 minutes ago.

Sound travels in air at about 340 metres per second – this is about a million times slower than light.

This is why fireworks exploding in the distance are seen long before they are heard. It takes sound about three seconds to travel a kilometre. Therefore fireworks exploding a km away will be heard 3 seconds after being seen.

a) Light Travels in Straight Lines

Punch tree small holes in the centre of three cards. Place a thread through the holes of the three cards.

Tie one end of the thread to a stand. Pull the thread loosely so that the three holes in the card at not in a straight line. Look through the first hole. The stand cannot be seen. Now pull the string tightly so the three holes are in a straight line. Look through the first hole – the stand can be seen. Now move one of the cards out of position and look through the first hole. The stand cannot be seen. Conclusion: light must travel in a straight line as it is only when the holes are in a straight line that objects behind the cards can be seen.

b) Explain How Shadows Are Formed

A shadow is a space or area of total or partial darkness caused by an opaque object blocking light source. Shadows with sharp edges are the result of light travelling in a straight line. Allow an opaque object block light from a very small opening. The shadow on the wall or screen is very sharp and dark. It is only because light travels in straight lines that the shadow is sharp.

The central dark full shadow space (umbra) is not receiving light from any part of the source that is facing it.

The outer partial shadow space (penumbra) is receiving light from only a part of the light source facing it. The shadow cast on the Earth during a solar eclipse is a double shadow. The Sun is million times larger than the Earth.

Colour is the brain’s response to the frequency of the light waves entering our eyes. Different frequencies produce different colours. The full spread or band of 7 colours is called the visible spectrum. The order of colours is: Red Orange Yellow Green Blue Indigo Violet Light from the Sun is often described as white light and is a mix of all the 7 colours of the visible spectrum. The Visible Spectrum of white light can be shown in the laboratory by passing a narrow beam of white light through a corner of a prism. A prism is a triangular block of transparent material e.g glass, quarts, perspex. The prism separates the colours by bending each over a different angle at entry and exit. The path of red is changed the least, violet the most. The separation of light into the 7 colours is called dispersion. The natural dispersion of light by raindrops makes a rainbow.

Reflection and Refraction Of Light

Reflection is the bouncing of light from a surface or boundary. The reflection of light from a plane surface is described as regular reflection. A plane surface is a perfectly flat surface – it is not a curved mirror so the light will bounce off the surface at the same angle that it hits the surface. A mirror is typically a thin block of glass with a very thin layer of silver on one side. The silver layer is protected with a layer of paint. The silver layer is the reflecting surface. (If the surface is not perfectly smooth but a bit rough then the light will bounce off in many different directions – this is called irregular reflection).

Image in a Plane Mirror The picture in the mirror is called an image. The image in a plain mirror is described as laterally inverted – the left side of the object appears as the right side in the image and vice versa.

Applications of Reflection

a. Investigate the Refection of Light by Plane Mirror: illustrate using ray diagrams.

Place two pins in line at an acute angle e.g. 45 ° to a mirror. Place another two pins so that they are in line with the reflections in the mirror of the first two pins. Mark the positions of the mirror and pins on the paper using a pencil. Draw straight lines through the two pairs of pin marks. Note again that the angle of incidence is equal to the angle of reflection.

b. Demonstrate and Explain the Operation of a Simple Periscope.

The two mirrors are held by the clamps, on a stand, at an angle of 45° facing each other and about 30 cm apart.

View the object beyond an obstacle by looking into the lower

mirror. Note that the image is (i) right way up, (ii) no lateral inversion, (iii) appears lower and (iv) further back than the object . Explanation. Because each mirror is set at 45° then the reflection at each mirror changes the direction of light by 90°, a right angle. After the second reflection the light is travelling in the same direction, parallel to its original direction.

Refraction is the bending of light when it passes from one medium into another. The change in direction takes place at the boundary. The light travels in a straight line in each medium.

Refraction of Light as it Passes a) From Air to Glass b) From glass to air.

The above can be demonstrated using two pairs of pins on opposite sides of the clear glass block such that on viewing the pins through the glass the four pins appear in a straight line. On passing from air to glass the light is bent towards the normal – the normal is the vertical line drawn at the point where the light meets the boundary between the air and glass. On passing from glass to air the light is bent away from the normal. The change in direction is due to a change in the speed of light.

Refraction by Lenses

An optical lens is a specially shaped transparent material with one or both of its surfaces curved to refract light in useful way. Two common types of lenses are convex and concave. The surface of a convex lens curves outwards like the exterior of a circle. A convex lens is thicker in the centre than at the edge. The concave lens curves like the interior of a circle. A concave lens is thinner in the centre. The effect of a lens is best demonstrated using parallel beams of light from a ray box.

Refraction by a Convex Lens: light rays brought closer together.

Refraction by a Concave Lens: light rays are spread further apart.

Demonstrate the Operation of a Magnifying Glass

A magnifying glass is a convex lens. Hold the magnifying lens between your eye and the object. Move the lens close to the object. At a close distance the object will appear larger. Move the lens back and forth to get the position at which the object’s image is greatest. The image of the object is on the same side of the lens as the object, is right way up, it is not inverted laterally, is larger than the object and appears behind the object.

The magnification depends on the curvature – the more rounded the lens the greater the magnification.

A magnifying lens is sometimes called a simple microscope.

Applications of Refraction

1. Convex Lens: magnifying glass, microscope, camera, vision – lens of the eye, spectacles for longsight.

2. Concave Lens: door spy-hole, opera glasses, spectacles for shortsight.

3. Dispersion of white light – producing the visible spectrum using a prism.

4. Use of a magnifying lens as a ‘reading glass’.