# [Keywords] Light

### Important Facts That You Must Know First!

1. Light travels at 3 x 10m/s in vacuum. In air, we take the speed of light to be the same value as when it is travelling in vacuum.
2. When light enters optical media such as glass, water and so on, the average speeds of light will be different (lower than 3 x 10m/s) . That’s why n = c/v where v represents the average speeds of light in the medium but most of the time it is just simplified to be speed of light in the medium. The reason is due to the time delay between absorption and re-emission of photon from one atom to another. However, the frequency of light before absorption and after re-emission still remains the same. Figure under point 3 illustrates how light travels through a pane of glass which results in the lower average speeds. Talk to your teacher if you are unsure of what I am saying here since this is a brief comment of fact that you should know.
3.  The principle of reversibility states that a ray of light will take the same path if its direction of travel is reversed.
4. Smooth surface will produce regular reflection while rough surface will produce irregular reflection (a.k.a diffused reflection).
5. We see an object because light rays that incident on the object reflected into our eyes. Our eyes cannot produce light ray like superman!
6. ROY Got Bullied In Vietnam = ROYGBIV
7. Red Meat Is Very Useful Xtremely Grilled = Radio, Microwave, Infrared, Visible Light, Ultraviolet, X-ray, Gamma Ray
8. There are TWO criteria that must be met in order for Total Internal Reflection (TIR) to take place: first criterion is the angle of incident must be greater than the critical angle and second criterion (a lot of people tend to forget) is that the light ray must be travelling from a region of higher refractive index to a region of lower refractive index.
9. In ray diagram drawing, there are THREE principal rays that you know how much they bend/refract after passing through the lens. However, you only need to draw 2 in order to deduce the location of the image. Take note that amongst the three principal rays, there is one ray that passes through the optical centre, C, of the lens that we draw as if the ray does not refract at all. That’s is the correct way for ray diagram, however, you should know that in real life, this ray obeys the law of refraction also. It is just that the misalignment of light rays due to the refractions at the front and back sides of the lens can be neglected.
10. Real image is an image that can be captured on a screen whereas virtual image cannot be captured on a screen.
11. Changing the size of the lens (in terms of its diameter while the curvature remains the same) only changes the brightness of the image formed and not cut off part of the image. The bigger the lens, the brighter the image formed and the image will become dimmer when the lens is smaller.
12. It is useful to know Snell’s law. It is extremely useful because it can be used for any combinations of the media and we do not need to remember which sine is on top.