Light is a form of energy that enters our eyes and allows us to see things.
Laws of reflection:
- The incident ray, reflected ray and the normal at point of incidence all lie in the same plane.
- Angle of incidence = Angle of reflection
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| Reflection and refraction of light |
Reflected ray: The light that is reflected off the surface
Normal: Imaginary line that is perpendicular to the surface at the point of incidence 'O' used even in curved surfaces
O, point of incidence: The point where the incident ray strikes the surface
i, angle of incidence: Angle between incident ray and normal
r1, angle of reflection: Angle between reflected ray and normal
Specular reflection and Diffused reflection
Mirrors
The picture of yourself formed in teh mirror is because light rays reflect off you, strike the mirror and travel to our eyes.
| Mirror Type | Object Position | Image Position | Image Type | Image Orientation | Image Size |
| Plane | Anywhere | On the opposite side of the mirror as the object | Virtual | Same as object | Same as object |
| Convex | Anywhere | On the opposite side of the mirror as the object | Virtual | Same as object | Diminished |
| Concave | Inside F | On the opposite side of the mirror as the object | Virtual | Same as object | Magnified |
| Exactly at F | At infinity | Virtual | - | Same as object | |
| Outside F, object distance < image distance | On the same side of the mirror as the object | Real | Opposite of object | Magnified | |
| Outside F, object distance = image distance | On the same side of the mirror as the object | Real | Opposite of object | Same as object | |
| Outside F, object distance > image distance | On the same side of the mirror as the object | Real | Opposite of object | Diminished |
Image formation in a mirror
Light rays from an object can take many different paths and if our eyes intercept some of these rays that are reflected from the mirror, they seem to come from the same distance behind the mirror. The image in the mirror is virtual because only our eyes perceive the object to be behind the mirror and if we put a screen at where the image is perceived to be, we would not be able to capture it.
Behaviour of light when it enters a different medium
At the interface, light is partially reflected off the surface and partially transmitted through the medium. The light that is transmitted through the medium bends as it travels from 1 optical medium to another, this is known as refraction.Within the same medium, light still travels in a straight line. Also, the higher the optical density of a medium, the lower the transmittance, thus the greater is the refraction of light in the medium.
When a light ray strikes perpendicularly to the surface of a medium it passes straight through without refraction as its angle of incidence is 0 so its angle of refraction is 0 as well. However its speed still decreases when optically denser mediums like glass.
It is also good to note that the angle of refraction is always smaller than the angle of incidence. This means that when a beam of light passes from an optically less dense medium into a denser optical medium, the ray of light bends toward the normal as light travels slower in the optically denser medium.
From this we can tell that the path light takes when it travels between air to glass and glass to air, the path it takes is the same, this is called the reversibility of light.
But in order to calculate the relation between the angle of incidence and the angle of refraction, the by product of this is the refractive index which is used to calculate angle of refraction from angle of incidence.
Using this law, we can also calculate the critical angle of glass which is when the ray of light travels exactly along the surface of glass, its angle of refraction is 0.
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| total internal reflection |
When angle of incidence is greater than critical angle and incident ray is in the optically denser medium, all the light is reflected as though it had hit a perfect mirror and there is no refraction at all. This is known as total internal reflection.
This concept can be used in optical fibres where light undergoes various rounds of total internal reflection to reach the other end of the optical fibre.
Total internal reflection is used in various methods:
- Transmit telephone and television signals
- Operate periscopes
- Endoscopes for medical uses
Lenses
Convex lenses are thinner at both edges and thicker at the centre, they are also known as converging lens as they converge light rays together. Like this, images formed by a convex lens are real as they meet at the image position and this image can be captured on a screen. Depending on the curvature of the lens, the parallel beam will converge at different focal points of a lens.
Terms used in ray diagram for lenses:
- Optical centre (point at centre of the lens)
- Principal axis (line passing symmetrically through optical centre of lens)
- Principal focus (point where all rays close to and parallel to principal axis converge after refraction by lens
- Focal length (distance between optical centre and focal point)
- Focal plane (plane passing through focal length and perpendicular to principal axis)
Concave lens
Concave lenses differ from convex lenses as they are thicker at both edges while thinner at the centre. They are also known as diverging rays because it diverges light rays that pass through the lens away from the focal point.
Dispersion of light
If we put a prism in the path of a beam of white light, we will be able to split the beam into 7 bands of colour known as the visible spectrum. It consists of red, orange, yellow, green, blue, indigo, violet. The process of splitting the light is known as dispersion of light. This is how rainbows are formed as light enters the raindrop, it goes through total internal reflection in the raindrop before it is refracted out into the 7 bands of light. This means that all the colours are refracted differently and a rainbow is formed.
Each light has its different wavelength and this results them in having different rates of slowing down and their angles of refraction are thus different as well. Red light has the longest wavelength at 700 mn and thus travels the fastest. On the other hand, violet light has the shortest wave length and thus travels the slowest. By putting another identical inverted prism next to the first 1, we are able to get the colours to combine together again into a white beam of light. This is known as the recombination of light.
Colours of light
Concave lenses differ from convex lenses as they are thicker at both edges while thinner at the centre. They are also known as diverging rays because it diverges light rays that pass through the lens away from the focal point.
Dispersion of light
If we put a prism in the path of a beam of white light, we will be able to split the beam into 7 bands of colour known as the visible spectrum. It consists of red, orange, yellow, green, blue, indigo, violet. The process of splitting the light is known as dispersion of light. This is how rainbows are formed as light enters the raindrop, it goes through total internal reflection in the raindrop before it is refracted out into the 7 bands of light. This means that all the colours are refracted differently and a rainbow is formed.
Each light has its different wavelength and this results them in having different rates of slowing down and their angles of refraction are thus different as well. Red light has the longest wavelength at 700 mn and thus travels the fastest. On the other hand, violet light has the shortest wave length and thus travels the slowest. By putting another identical inverted prism next to the first 1, we are able to get the colours to combine together again into a white beam of light. This is known as the recombination of light.
Colours of light
- Primary colours= red,blue,green
- Secondary colours= yellow, cyan, magenta
- Yellow= red+green
- Cyan= green+blue
- Magenta= red+blue
When lights are shone on transparent materials such as filters, they selectively absorb 1 or more colours and transmit what is not absorbed.
Contrary to transparent materials, opaque materials selectively absorb 1 or more colours and reflect what is not absorbed.
Reflections
This term was about sexual reproduction in humans and physics involving light and lenses. I felt that it was quite a good move for Ms Nada to allow us to ask her any questions we might have before we started the topic so as to allow us to get used to the awkward phrases like sex. Other than learning about how our body is adapted for reproduction, we also learnt about the various contraceptives available in the market, many of which I have not heard about before. However I felt that the section on STDs was quite disgusting due to the graphic pictures. In physics, the law of how angle of incidence=angle of reflection surprised me as I always thought that the angle light reflected off everything was 90 degrees. We were also taught shortcuts on how to draw ray diagrams with or without lenses and why something underwater may not actually be at the position we see it. I learnt that a rainbow is formed by total internal reflection and how light is made out of different colours that travel at different speeds. In this test, I got 30/45 because I did not draw the ray diagrams properly, a big mistake which I hope to rectify in the EOY.
This term was about sexual reproduction in humans and physics involving light and lenses. I felt that it was quite a good move for Ms Nada to allow us to ask her any questions we might have before we started the topic so as to allow us to get used to the awkward phrases like sex. Other than learning about how our body is adapted for reproduction, we also learnt about the various contraceptives available in the market, many of which I have not heard about before. However I felt that the section on STDs was quite disgusting due to the graphic pictures. In physics, the law of how angle of incidence=angle of reflection surprised me as I always thought that the angle light reflected off everything was 90 degrees. We were also taught shortcuts on how to draw ray diagrams with or without lenses and why something underwater may not actually be at the position we see it. I learnt that a rainbow is formed by total internal reflection and how light is made out of different colours that travel at different speeds. In this test, I got 30/45 because I did not draw the ray diagrams properly, a big mistake which I hope to rectify in the EOY.
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