*Please click video title headings to play video if video does not load properly.
Basic properties of light
1. Light travels in straight line.
2. Light is a form of energy.
3. Speed of light is approximately 3 X 10^8 m/s.
4. Light does not require a medium to travel. It can travel through vacuum.
2. Light is a form of energy.
3. Speed of light is approximately 3 X 10^8 m/s.
4. Light does not require a medium to travel. It can travel through vacuum.
Laws of reflection of light
1. At the point of incidence, the incident ray, the reflected ray and normal all lie in the same plane.
2. The angle of incidence is equal to the angle of reflection.
The following video shows an experiment to demonstrate that angle of incidence is always equal to angle of reflection. Also, note that the first law about the incident ray, reflected ray and normal all lie in same plane is also demonstrated. In the experiment, we are able to see the reflected ray precisely because it lies on the same plane with the incident ray.
2. The angle of incidence is equal to the angle of reflection.
The following video shows an experiment to demonstrate that angle of incidence is always equal to angle of reflection. Also, note that the first law about the incident ray, reflected ray and normal all lie in same plane is also demonstrated. In the experiment, we are able to see the reflected ray precisely because it lies on the same plane with the incident ray.
Characteristics of image formed by a plane mirror
1. It is the same size as the object.
2. It is formed at same distance behind the mirror as the object is in front of the mirror.
3. It is laterally inverted.
4. It is virtual (A virtual image is an image that cannot be formed on a screen).
See following video which shows an experiment to demonstrate some of the characteristics of the image such as being same size as object, same distance behind mirror as the object is in front of mirror and image being laterally inverted.
2. It is formed at same distance behind the mirror as the object is in front of the mirror.
3. It is laterally inverted.
4. It is virtual (A virtual image is an image that cannot be formed on a screen).
See following video which shows an experiment to demonstrate some of the characteristics of the image such as being same size as object, same distance behind mirror as the object is in front of mirror and image being laterally inverted.
How to draw ray diagram for reflection
Refraction of light (part 1)
Refraction of light is the bending of light as it travels from one transparent medium to another different transparent medium.
When light travels from an optically less dense medium (air) to an optically denser medium (glass), its speed decreases and it is refracted towards the normal.
When light travels from an optically denser medium (glass) to an optically less dense medium (air), its speed increasess and it is refracted away from the normal.
In this video, it introduces the idea of refraction.
When light travels from an optically less dense medium (air) to an optically denser medium (glass), its speed decreases and it is refracted towards the normal.
When light travels from an optically denser medium (glass) to an optically less dense medium (air), its speed increasess and it is refracted away from the normal.
In this video, it introduces the idea of refraction.
Refraction of light (parts 2 and 3)
In the following two videos, they explore how the formula (Refractive index, n = sin i / sin r) is formed.
**It is very important to take note that the formula n = sin i / sin r only applies to light travelling from air to glass! The refractive index n we are finding is that of glass. However, if light is travelling from glass to air, we have to invert the formula to get n = sin r / sin i whereby the n is still refractive index of glass.
Please see following videos to know why we have to invert the formula for the case when light travels from glass to air! Do not just blindly memorise one formula and assume everything is using the same formula. It always pays to understand things well!
Follow this link to part 2 video.
**It is very important to take note that the formula n = sin i / sin r only applies to light travelling from air to glass! The refractive index n we are finding is that of glass. However, if light is travelling from glass to air, we have to invert the formula to get n = sin r / sin i whereby the n is still refractive index of glass.
Please see following videos to know why we have to invert the formula for the case when light travels from glass to air! Do not just blindly memorise one formula and assume everything is using the same formula. It always pays to understand things well!
Follow this link to part 2 video.
Refraction of light (part 4)
Refraction of light (using the formula refractive index, n = c / v)
So far from above, we know that refractive index of glass, n = sin i / sin r (if light is travelling from air to glass)
n = sin r / sin i (if light is travelling from glass to air)
The following two videos explore another formula refractive index of medium, n = c / v.
c is the speed of light in air/vacuum which is 3 X 10^8 m/s while v is speed of light in the medium.
Follow this link to first video.
n = sin r / sin i (if light is travelling from glass to air)
The following two videos explore another formula refractive index of medium, n = c / v.
c is the speed of light in air/vacuum which is 3 X 10^8 m/s while v is speed of light in the medium.
Follow this link to first video.
Simple worked example on using the formula n = sin i / sin r
Take note that in this given example, the angle that is given is the angle the light made with the horizontal surface which is 30˚. We need to get the angle of incidence by taking 90˚- 30˚= 60˚. Some students when rushing through a question never read the question properly and just use the angle they see in the question thinking it must always be the angle of incidence. Be careful when reading question. Prudence is an important attitude one must have. Do not be impulsive and careless!
Laws of refraction of light
1. At the point of incidence, the incident ray, refracted ray and normal all lie in the same plane.
2. When light is travelling from air to a denser medium, the angle of incidence and angle of refraction are related by the ratio sin i / sin r = n whereby n is the refractive index of the denser medium.
2. When light is travelling from air to a denser medium, the angle of incidence and angle of refraction are related by the ratio sin i / sin r = n whereby n is the refractive index of the denser medium.
Simple experiment to show refraction of light
Total internal reflection
This video explores critical angle, total internal reflection and formula sin C = 1/ n.
Do take note that total internal reflection can have a possibility of happening ONLY when light is travelling from a denser medium to a less dense medium (for O level the less dense medium is always given as air).
Critical angle is the angle of incidence of light travelling in the denser medium in order to have an angle of refraction of 90˚ in the less dense medium.
When light is travelling at angle of incidence greater than critical angle, total internal reflection occurs.
sin C = 1 / n. C is critical angle and n is the refractive index of denser medium.
The calculation shown in this video is using this formula sin C = 1 / n for finding critical angle. If you got a little bit confused over the beginning part about applying refractive index of the two media using Snell's Law, just ignore that part and look at the last few steps which resembles sin C = 1 / n. The refractive index of water n in this video is 1.33 being substituted into the formula for finding critical angle.
I provide another simple way to appreciate the question asked in this video if you find the question asked on finding critical angle a bit confusing.
Qn asked: Find the critical angle for light travelling in the water. Take refractive index of water n be 1.33.
Ans: Applying the formula sin C = 1/ n , we will substitute the value of n to be 1.33 and just solve the formula to find angle C, the critical angle. This is basically what the question in this video is talking about.
Do take note that total internal reflection can have a possibility of happening ONLY when light is travelling from a denser medium to a less dense medium (for O level the less dense medium is always given as air).
Critical angle is the angle of incidence of light travelling in the denser medium in order to have an angle of refraction of 90˚ in the less dense medium.
When light is travelling at angle of incidence greater than critical angle, total internal reflection occurs.
sin C = 1 / n. C is critical angle and n is the refractive index of denser medium.
The calculation shown in this video is using this formula sin C = 1 / n for finding critical angle. If you got a little bit confused over the beginning part about applying refractive index of the two media using Snell's Law, just ignore that part and look at the last few steps which resembles sin C = 1 / n. The refractive index of water n in this video is 1.33 being substituted into the formula for finding critical angle.
I provide another simple way to appreciate the question asked in this video if you find the question asked on finding critical angle a bit confusing.
Qn asked: Find the critical angle for light travelling in the water. Take refractive index of water n be 1.33.
Ans: Applying the formula sin C = 1/ n , we will substitute the value of n to be 1.33 and just solve the formula to find angle C, the critical angle. This is basically what the question in this video is talking about.
Experiment to show total internal reflection through a coiled plastic tubing
This experiment demonstrates the effect of total internal reflection similar to that used in fibre optics.
The advantages of using fibre optics are that it is cheap, it can transmit more information and the information can travel at faster speed than using electric cables.
The advantages of using fibre optics are that it is cheap, it can transmit more information and the information can travel at faster speed than using electric cables.
Thin converging lens (object distance more than 2F)
See following video on drawing the light rays for object distance more than 2F.
The image produced here is inverted, real and diminished (smaller size than object). This example shows how we use a thin converging lens in a camera. Common sense tells us that the image formed inside a camera is sure to be diminished (much smaller size) compared to the actual size of object taken, thus this example must be referring to image produced in a camera.
For all the uses of thin converging lens in this section, one must use common sense to understand the type of image formed and thus their respective use. Rely on common sense and understanding as always, not blind memory work without any understanding.
The image produced here is inverted, real and diminished (smaller size than object). This example shows how we use a thin converging lens in a camera. Common sense tells us that the image formed inside a camera is sure to be diminished (much smaller size) compared to the actual size of object taken, thus this example must be referring to image produced in a camera.
For all the uses of thin converging lens in this section, one must use common sense to understand the type of image formed and thus their respective use. Rely on common sense and understanding as always, not blind memory work without any understanding.
Thin converging lens (object distance between F and 2F)
The image produced here is inverted, real and magnified (larger size than object).
Can you guess the use for this example by using common sense? Of course, this way of using thin converging lens is in a projector. The image is magnified and real just as that of the image formed by a projector which is large and can be formed on a screen (real image). Again, nothing mysterious about this. Common sense again.
Can you guess the use for this example by using common sense? Of course, this way of using thin converging lens is in a projector. The image is magnified and real just as that of the image formed by a projector which is large and can be formed on a screen (real image). Again, nothing mysterious about this. Common sense again.
Thin converging lens (object distance less than F)
The image produced here is upright, magnified and virtual (image formed on the same side as the object; note that such virtual image cannot be formed on a screen).
Using common sense, we can figure out that since the image is magnified and upright, this way of using thin converging lens is in a magnifying glass. Surely the image you see based on past experience using a magnifying glass is magnified (large) and also upright (not inverted) right?
Using common sense, we can figure out that since the image is magnified and upright, this way of using thin converging lens is in a magnifying glass. Surely the image you see based on past experience using a magnifying glass is magnified (large) and also upright (not inverted) right?
Thin converging lens (other examples)
No videos on these other examples. You may refer to your own school notes, textbooks or assessment books for these other examples. The best way is still to draw out your own ray diagram to figure out what type of image is formed and then guess the use of the lens using common sense. No short-cut to learning. Just have to try out and make mistakes and relearn the correct way of doing things to master the thing one is learning.
Object distance at 2F: Image formed is inverted, real and same size as object. This way of using thin converging lens is in a photocopier <image is same size as object much like a photocopier making exact same copy and size of image as object>.
Object distance is at infinity: Image formed is real and at F. Since object is at infinity, this way of using thin converging lens must be in a telescope for viewing distant object.
Object distance is at F: Image formed at infinity (light rays travel parallel to each other after passing through lens). This way of using thin converging lens which project parallel light rays must be in a spot light which project parallel light rays for very long distances (image is formed at infinity).
Object distance at 2F: Image formed is inverted, real and same size as object. This way of using thin converging lens is in a photocopier <image is same size as object much like a photocopier making exact same copy and size of image as object>.
Object distance is at infinity: Image formed is real and at F. Since object is at infinity, this way of using thin converging lens must be in a telescope for viewing distant object.
Object distance is at F: Image formed at infinity (light rays travel parallel to each other after passing through lens). This way of using thin converging lens which project parallel light rays must be in a spot light which project parallel light rays for very long distances (image is formed at infinity).