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Learn about light energy, shadows, reflection, refraction, and the electromagnetic spectrum. Design investigations, understand periscopes and lens usage for eye disorders.
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At the end of this unit you should: 1. Know that light is a form of energy and that it can be converted to other forms. 2. Be able to design an investigation demonstrating that light travels in straight lines. 3. Understand how shadows are formed. 4. Understand the difference between luminous and non-luminous objects. 5. Understand that white light is made up of different colours which can be separated by dispersion. 6. Be able to produce white light, a spectrum of light and list the colours of the spectrum.
At the end of this unit you should: 7. Understand the operation of a simple periscope. 8. Be able to design a simple periscope. 9. Understand reflection and refraction. 10. Be able to describe how concave and convex lens can be used in glasses to resolve common eye disorders. 11. Be able to identify the regions on the electromagnetic spectrum and briefly describe each region.
concave convex dispersion electromagnetic spectrum gamma rays image infrared luminous medium microwaves non-luminous normal periscope perpendicular line prism radio waves ray reflection refraction shadow spectrum ultraviolet light visible light wave x-ray
LIGHTBULB QUESTION The water blurs/changes the light, which makes the straw appear to bend.
Light: Light is a form of energy emitted by hot bodies, which travels in straight lines. It is the only form of energy which we can indirectly see. Luminous Object: An object that produces its own light. Non-luminous Object: An object that does not produce its own light. It can reflect light from luminous objects.
Can you give other examples of luminous and non-luminous objects? Luminous: Sun, lightbulb, torch, a switched-on television. Non-luminous: Moon, mirror, table with a lacquered surface.
Investigation 11.03.01: How light travels Equipment: Lightbulb, three or more pieces of cardboard with hole in centre, Blu-tack.
Investigation 11.03.01: How light travels Instructions: 1. Pierce a hole in the centre of each piece of cardboard. 2. Arrange the cardboard so that you can see through each hole to the other side. 3. Place the lamp on one side of the three pieces and look through the holes. 4. Move one of the pieces off centre and look through again.
1. How did you ensure this was a fair test? • Justify your answer. • Each hole was the same size and in the same place on each piece of cardboard. • 2. Does light bend around corners? Explain your answer. • No, it does not, as we could not see the light when one of the pieces was moved off centre. • In a similar way you cannot see what is directly behind the door of the classroom, or what is around the corner of the hall.
3. Would your conclusion change if the holes in the card were bigger? Why? • No. If the holes were bigger, more light would be captured, but if one of the holes was off centre we would still not see the light.
Shadow: The darkness behind an object when light cannot pass through it.
(a) True or False: Light is a form of energy. • True. (b) How can you justify that light is/is not a form of energy? • Light is a form of energy as you can feel the heat from the light of the sun. You can also feel the heat from the light of a bulb. Also, light can be used in solar cells to power the heating systems in homes and generate electricity.
(c) Distinguish between luminous and non-luminous objects. • Luminous objects produce their own light whereas non-luminous objects just reflect the light. (d) Give the definition for a shadow. • When light cannot pass through an object, the darkness behind that object is called a shadow.
Ray of Light: A single beam (column) of light.
Investigation 11.03.02: The principle of reflection Equipment: Mirror, light source, paper, protractor, pencil.
Investigation 11.03.02: The principle of reflection Instructions: 1. Set up the mirror so that it is standing upright on a piece of paper. Draw the outline of the mirror on the page so you have a starting point again if the mirror falls over. 2. Using the light source, shine the light at an angle towards the centre of the mirror. 3. Using a pencil, draw along the line of the light to the mirror and the line reflected off the mirror. 4. Using a protractor, measure the angle created between the mirror and the incident ray. 5. Using a protractor, measure the angle created between the mirror and the reflected ray. 6. Compare these angles. 7. Repeat the steps for other angles of incidence.
Does a relationship exist between the incident ray and the reflected ray? Explain your answer. • Yes. The angle of incidence is equal to the angle of reflection. • 2. Suggest one change you would make to improve the accuracy of this investigation. • One improvement could be using a ruler to draw the lines of light since light travels in straight lines.
Reflection: The bouncing of light from a surface.
Refraction: The deflection of light at the surface/interface as it passes from one medium into another. Medium: A solid, liquid or gas which allows waves to travel through it.
Investigation 11.03.03: The effect of refraction of light Equipment: Glass block, laser, paper, protractor, pencil. 1 2 3
Investigation 11.03.03: The effect of refraction of light Instructions: 1. Place the glass block on a sheet of white paper and trace the outline of the block. This is done for two reasons: 1) It gives you your starting point should the block be moved, etc.; 2) It gives you an entry and exit point for the light, which you can join through the block to see how light has refracted. 2. Place the light at an angle on one side of the block and trace the incident ray to the block. 3. Trace the refracted ray at the other side of the block. 4. Remove the block from the paper and join the incident ray and refracted ray through the block. 5. Using a set square, draw the normal from the angle of incidence.
1. What was the angle of refraction as the light entered the glass block? • Calculate and note the value as you conduct the investigation. • 2. What was the angle of refraction as the light exited the glass block? • Calculate and note the value as you conduct the investigation.
3. If possible, can you describe the relationship between the angles of refraction (entering and leaving the block)? • The angle of incidence should be equal to the angle of refraction. • 4. Conduct this investigation again using a beaker of water instead of a glass block. What do you expect to happen? • The results should be identical.
(a) Give the definition of reflection. Reflection is the bouncing of light from a surface. (b) Give the definition of refraction. Refraction is the deflection/bending of light as it passes from one medium into another. (c) How are shadows formed? When light cannot pass through an object, the darkness behind that object is called a shadow.
(d) What is the name given to the ray coming from the light source heading towards a medium? Incident ray. (e) Where can you find the reflected ray? It is found reflected off the surface where the incident ray hits.
(f) Name the two types of lens and the eye disorders they are used to resolve. A concave lens causes the light to diverge and is used to resolve short-sightedness. A convex lens focuses the light into one area and is used to resolve long-sightedness. (g) Where in the eye do we want the image to form? The retina.
LIGHTBULB QUESTION The light creates a rainbow effect on the CD. This is due to dispersion of the white light (sun/ lightbulb) into its constituent colours.
Spectrum: A band of colours produced by separation of the components of light by their different degrees of refraction according to wavelength. Dispersion: The breaking up of white light into its constituent colours.
Investigation 11.03.04: The dispersion of white light Equipment: Glass prism, ray box, paper. Instructions: Shine the light through the prism and turn the prism until a rainbow effect is seen on the sheet of paper.
1. Can you produce a spectrum every time you try? • Yes, the prism is able to split the light into its colours. • 2. Using the idea of refraction, can you explain (or draw) what happens to the light as it travels through the prism? • The light is refracted as it goes into the prism. This is due to the fact that different wavelengths of light diffract at different angles.
LIGHTBULB QUESTION Light and sound energies can travel as waves.
Wavelength: The length of a wave before it starts to repeat itself. Crest to crest or trough to trough. Fig 11.03.19 is a diagram of a wave. One cycle (wavelength) is from one crest (top of wave) to the next crest.
Copy and Complete In this unit I learned that lightis a form of energy. All objects either produce their own light while others do not and just reflect it. Objects which produce their own light are called luminous, while those which don’t are called non-luminous. Light travels in a straight line. When light bounces off a surface it is called reflection. This is how we see our moon in the sky. Reflected light obeys this simple rule: the angle of incidence is equal tothe angle ofreflection. I also learned why a pencil appears to bendwhen placed in a beaker of water. This is due torefraction. This phenomenon occurs when light passes from onemediuminto another. When light passes from a less densemedium to a more densemedium, the light is refractedtowards the normal. When light passes from a more densemedium to a less densemedium, the light is refractedaway from the normal.
Copy and Complete Convexlenses are prescribed to long-sighted people as theyconvergethe light to one spot. Concavelenses are prescribed to short-sighted people as they divergethe light out. White light is made up of seven colours: red,orange,yellow,green,blue,indigo, and violet. The splitting of white light into its constituentcolours is called dispersion. The electromagneticspectrum contains the different wavelengths of light. At the right of the spectrum we have radiowaves. These waves can be as long as 10 km. One wavelength is measured fromcrest/troughtocrest/trough. As we go from right to left of the spectrum thewavelengthgets shorter and theenergy increases.
1. Draw a diagram explaining refraction. As light passes from a medium to a more dense medium, the light refracts (bends [towards the normal]).
2. What is the rule for reflection? • Angle of incidence = angle of reflection.
3. What is wavelength? Wavelength is the distance from one crest to the next crest in a wave.
4. Where in the electromagnetic spectrum do you find x-rays? X-rays are found in the x-ray region of the EM spectrum.
5. Give an example of where you can find x-rays in your everyday life. Hospitals/airport security.
6. What is the angle at which the incidence ray hits the mirror? Angle of incidence.
7. What is the angle at which the reflected ray leaves the mirror? Angle of reflection.
