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Light.

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Presentation on theme: "Light."— Presentation transcript:

1 Light

2 What is light?

3 What is light? Light is produced by the Sun, and by objects such as light bulbs and matches. Light carries energy and travels as a wave. Light travels at a speed of 300,000,000 metres per second, which is much faster than the speed of sound. Photo credit: © 2008 Jupiterimages Corporation Light waves travel in straight lines.

4 Light energy Light is a form of energy and can be changed from one form into another. Light energy can be used to make other useful forms of energy. It can be converted into electrical energy in a solar cell or chemical energy in the leaves of plants. Photo credit: Courtesy of BP Solarex/NREL Telecom installation of a local utility company uses solar cells to measure water flow to Casablanca, Morocco. solar cell plants

5 Which materials let light through?

6 How do we see things? An object that gives out light is described as luminous. How does light from a luminous object such as a light bulb reach the eye? Light travels in a straight line directly into the eye. An object that does not give out light is non-luminous. How does your eye see a non-luminous object such as a comb? Light hits the comb and some of it is reflected into the eye.

7 How the eye works optic nerve – carries information from the eye to the brain retina – where the focused light is detected ciliary muscle – where light enters the eye lens – light is focused onto the retina iris – controls the amount of light entering the eye cornea – focuses the light and protects the eye

8 Reflection

9 Reflective materials Objects that reflect light well:
Have smooth, shiny surfaces and are usually pale colours. Give clear images because they reflect light regularly. Objects that do not reflect light well: Have rough, matt surfaces and are usually dark colours. Give diffuse images (or do not give any images) because they reflect light irregularly. This is called scattering. Photo credit (top): © 2008 Jupiterimages Corporation Photo credit (bottom): © 2008 Jupiterimages Corporation

10 Reading in mirrors Working in pairs, decide who is the ‘timer’ and who is the ‘reader’. The ‘reader’ has to read a selection of words reflected in a mirror. They must read each word correctly before moving on to the next. The ‘timer’ measures the time taken and the results for the whole class are recorded in a table like this: Time taken (s) Natasha Shani Rajesh Name 1. Why are the words so difficult to read in the mirror – how do they appear? 2. What was the average time taken in the class? 3. Plot a bar chart of results. Worksheet 1 accompanies this slide. The worksheet includes a cut-out word card and some follow-up questions that test understanding.

11 What is lateral inversion?
A plane mirror reflects light regularly so it produces a clear image, which is the same size as the object. The image appears the same distance behind the mirror as the object is in front of it. What is different about the image compared to the object? When an object is reflected in a plane mirror, left appears as right and right appears as left. This type of reversal is called lateral inversion.

12 Reflection at a mirror

13 Reflection ray diagram

14 Reflection investigation
Fix a plane mirror to a piece of paper and draw around it. Draw a normal (a line at 90° to the mirror) through the centre of the mirror outline. Use a ray box to shine an incident ray at the mirror – plot the incident and reflected rays. angle of incidence [i] angle of reflection [r] Measure the angles of incidence [i] and reflection [r]. Repeat for another four angles of incidence. What do the results show?

15 Using reflection Reflection can be very useful.
High-visibility strips are very reflective and make sure that this cyclist gets noticed when there is little light. How does a periscope use reflection? The two plane mirrors must be positioned at 45° from the vertical. Light is reflected at right angles from the top mirror onto the bottom mirror and into the eye of the viewer.

16 Reflection: summary

17 Reflection: true or false?

18 Refraction

19 Refraction at an air-glass boundary

20 Refraction investigation
Place a rectangular glass block on a sheet of paper and draw around it. Draw a normal at 90° to the top surface of the block. Shine light rays, with angles of incidence [i] of 30°, 60° and 0°, into the block where the normal meets the glass surface. Record the angle of refraction [r] and the angle of the rays leaving the glass block [r2]. angle of incidence [i] angle of refraction [r2] angle of refraction [r1] 0 ° 30 ° 60 ° Worksheet 2 accompanies this slide. The worksheet guides students through conducting this experiment and provides follow-up questions.

21 Refraction in a glass block

22 Explaining refraction

23 Why is light refracted? The speed of light depends on the material through which the light is travelling. When light enters a different material (e.g. when moving from air into glass), the speed of light changes. This causes the light to bend or refract. air glass The speed of light is affected by the density of the material through which it is travelling. When light enters a more dense medium, its speed decreases and this is why refraction occurs.

24 Refraction ray diagram

25 What happens during refraction?

26 Effects of refraction Light from the part of the pencil in the water is refracted as it travels from the water into the air, making it appear bent. How does refraction make this stone look closer to the surface of the water than it really is? Light rays from the stone are refracted as they leave the water. The brain assumes the rays have travelled in straight lines, and is fooled into forming an image where it thinks the light rays came from.

27 Effects of refraction – the Archer fish
The archer fish is a predator that shoots jets of water at insects near the surface of the water. The archer fish allows for the refraction of light at the surface of the water when aiming at its prey. image of prey prey location The fish does not aim at the refracted image it sees but at a location where it knows the prey to be.

28 Refraction – true or false?

29 Colour

30 Passing white light through a prism

31 Richard Of York Gave Battle In Vain
Splitting white light A prism splits a ray of white light into the colours of the rainbow. This process is known as dispersion. The colours that make up white light are called the spectrum. Dispersion occurs because different colours of light refract differently. Red light refracts the least; violet light the most. The order of the colours in the spectrum is always the same. Use this phrase to remember the order of colours: Richard Of York Gave Battle In Vain Y

32 Natural dispersion If there are water droplets in the air and the sun is illuminating them from behind, then you may see a rainbow in the air. Light enters the water droplets and refracts. It then reflects off the back of the rain drop. The red light refracts the least and the violet the most. This causes dispersion of the sunlight. Photo credit: © 2008 Jupiterimages Corporation

33 The visible light spectrum

34 Recombining colours

35 Mixing coloured light

36 How do we see different colours?

37 Seeing different colours
How do we see the different colours in this frog and lily? The frog’s red skin absorbs all of the colours except red and so it appears red. The black skin absorbs all colours. No colours are reflected and so it appears black. The lily’s centre absorbs all colours except red and green. It reflects red and green light, and so appears yellow. The leaves reflect all the colours and so appear white. Photo credit (top): © 2007 Jupiterimages Corporation Photo credit (bottom): © 2007 Jupiterimages Corporation

38 Using filters of primary colours
A filter absorbs some colours of white light and lets other colours through to create coloured light. A red filter absorbs all colours… …apart from red light. A blue filter absorbs all colours… …apart from blue light. A green filter absorbs all colours... …apart from green light.

39 Using filters of secondary colours
A magenta filter absorbs all colours… …apart from red and blue. A cyan filter absorbs all colours… …apart from green and blue. A yellow filter absorbs all colours... …apart from red and green.

40 Seeing colours in coloured light

41 How do we see colours in coloured light?

42 The electromagnetic spectrum

43 Colour summary

44 Summary activities

45 Glossary absorption – When light is taken in by a material.
dispersion – The separating of the colours in light, e.g. when white light passes through a prism. image – A copy of an object formed when light is reflected from a mirror. light – A form of energy that is detected by the eyes. law of reflection – When light is reflected, the angle of incidence equals the angle of reflection. prism – A block of glass, usually triangular, which separates the colours in light. reflection – The bouncing back of light from a surface. refraction – The bending of light when it passes into a different medium, e.g. from air into glass. spectrum – The range of colours that make up white light.

46 Anagrams

47 Multiple-choice quiz


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