Topic outline
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Outcomes
By the end of this course you will:
- Be able to state what reflection is
- Be familiar with the different types of reflection
- Understand the laws of reflection
- Understand the concept of mirrors and reflection
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You will remember from the previous course titled 'Propagation and transmission of light', that the reason you can see non-luminous objects is because light from a light source e.g. sun or a light bulb is reflected off the non-luminous object.
The reflection of light occurs when a light ray bounces off an object. A good example is when light rays strike a smooth, polished surface like a mirror. These rays are not transmitted or absorbed, they bounce back and a reflection occurs. These light rays are referred to as reflected light rays.
The light rays that strike the surface are referred to as incident light rays.
Image: MaxPixel (PD)
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The image below clearly illustrates the difference between incident and reflection light rays. An interesting point to note is that the angle of incidence is equal to the angle of reflection - but more will be covered on this topic a little later!
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Regular / specular reflection
Regular, or specular reflection occurs when light rays bounce off a highly polished surface like a mirror. The reflected light is very clear and defined. Both the incident and reflected light rays are parallel.
Image: Wikimedia Commons (CC BY)
The above lake image would appear very different if the water were choppy due to the wind. The resulting reflection would then be diffuse.
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Irregular / diffuse reflection
Diffuse reflection occurs when light rays strike an irregular, or rough surface resulting in the reflected rays being reflected in different directions. Since the reflected rays are not parallel the result is either no image, or a distorted image being formed.
The image below clearly illustrates diffuse reflection. The water is slightly choppy resulting in a blurry, ill-defined reflection.
Image: HippoPx free photo (PD)
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Did you know?
Image: Raw Pixel (CC0)
Do you know why the sky is blue?
Sunlight contains many different colours ranging from blue to red. The blue light waves are shorter than the longer red waves. When sunlight enters the earth's atmosphere, the sunlight is scattered by the gas molecules which are present in the earth's atmosphere. Since the blue light waves are shorter than the red light waves, they are scattered more and in all directions resulting in a blue sky.
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So far, you have learnt about why reflection occurs and how it differs depending on the surface which the incident rays strike. Let's re-look at the image you studied earlier and define some reflection laws which it illustrates.
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Right angle mirrors
Refer to the image below as we discuss this topic.
Two mirrors are placed at 90 degrees to each other. Each of the plane mirrors produe a left-right reversal image. These images are referred to as primary images.
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But right angle mirrors actually produce three images.
The third image is formed by light rays which reflect off mirror 1 onto and off mirror 2 into your eyes. This third image is known as the secondary image. This secondary image is a result of two reflections and does not show the left-right reversal.
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Images in parallel mirrors
- Two mirrors are placed parallel to each other two metres apart
- A person stands in the middle between the two mirrors. There will be one metre between each mirror and the person
- When mirrors are exactly parallel (i.e. there is no angle between them), remember that the image distance equals the object distance
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Let's now take this a step further:
- Image 1 can also be considered as an object for mirror 2
- Image 1 will be 3m from mirror 2 and therefore the secondary image will be 3m behind mirror 2
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Based on the same principle, there will be a secondary image of image 2 which appears in mirror 1.
These secondary images also act as objects to create a third pair images, and so the process continues. Ultimately, an infinite number of secondary images are created.
When the mirrors are facing each, these images are not clearly seen since:
- Since there are so many reflections, the images become faint and are not clearly visible
- The human eye cannot comprehend far off images since the angle subtended by these images on the human eye is very small
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