Light
In this lesson, we learnt about LIGHT:
1) Reflection and Refraction
2) Lenses
3) Colours of light
Reflection & Refraction
Light travels in a straight line and thus, it cannot turn away or bend off course to avoid any object in its path. As light strikes the surface of different mediums, there will be reflection. Not all the light is absorbed by the object and some light is reflected off the object and into the surroundings. The key rule of reflection is that the angle of reflection is equal to the angle of incidence. Here is a diagram showing the law.
The dotted line in the middle of the figure is called the normal and is perpendicular to the surface. The properties of reflection is used in mirrors. So, why is it that we can see our reflection on a mirror but not on a piece of paper? This is because there are two different types of reflection: specular reflection and diffused reflection.
Specular reflection occurs when a light ray or a beam of light hits a smooth surface. Diffused reflection occurs when a light ray or a beam of light hits a rough surface and is reflected at different angles. A piece of paper has a seemingly smooth surface but it actually has millions of microscopic bumps and holes on it. This causes diffused reflection and thus, we cannot see our image on a piece of paper.
However, not all the light is reflected off the surface. What happens to the light which is absorbed?
Refraction is the process where light bends away from or towards the normal when it passes from a medium of higher density to a medium of lower density or vice versa. When light passes from an optically denser medium into an optically less dense medium, it refracts and bends away from the normal. When light passes from an optically less dense medium into an optically denser medium, it refracts and bends towards the normal. Here is what actually happens when a ray/beam of light strikes a surface:
The angle of refraction, as seen in the diagram, is the angle formed by the refracted ray and the normal. The greater the optical density of the medium, the greater/smaller the angle of refraction depending on the direction of light. In order to remember which way the light will bend, just imagine the light ray as a trolley which is being pushed across the floor at high speeds. The floor is the optically less dense medium. This trolley is being pushed at an angle. Now, imagine a carpet right in front of you. That is the optically denser medium, As one wheel of the trolley hits the carpet, it slows down. However, the other wheels of the trolley are still moving at high speeds. thus, the trolley begins to turn towards an imaginary line on the carpet, which is the normal.
Some rules to follow:
The angle of incidence is NOT equal to the angle of refraction
There is no angle of refraction if the angle of incidence is 90 degrees.
Lenses:
There are 2 types of lenses which we are studying. They are the concave lens and the convex lens. Before starting on the topic of lenses, we did an experiment to determine if the distance of an object infront of a mirror is equal to the distance of the image in the mirror. Here is our set-up as shown below:
The distance between L and the mirror was measured. Here is the image in the mirror:
As you can see, the image is upright, laterally inverted and same size as the object.
This topic on lenses is a very confusing topic and requires memorizing whole chunks of information. Here is a brief summary of what we learnt:
Optical center of a lens: point at centre of lens
Principal Axis: line passing symmetrically through the optical centre of the lens
Focal Point: Point where all rays close to and parallel to the principal axis converge after refraction by lens
Focal Length: Distance between optical centre and focal point
Focal Plane: Plane passing through the focal point and perpendicular to the principal axis
Here are all the cases in which light strikes a convex lens:
There are 6 cases in total.
Case Image Uses
1 ~real ~ telescope
~inverted
~laterally inverted
~diminished
2 -real -camera
-inverted
-laterally inverted
-diminished
3 *real *photocopier (equal sized copy)
*inverted
*laterally inverted
*same size as object
4 >real >projector
>inverted
>laterally inverted
>magnified
5 ! virtual ! spot light
! upright
! magnified
6 ~ virtual ~ magnifying glass
~ upright
~ magnified
Here is the case for a concave lens:
A concave lens is also known as a diverging lens because it diverges light rays that passes through the lens. The focal point, F, is the point where the diverging rays meet when extended backwards.
Here are some photographs of concave and convex mirrors:
Concave Mirror
Convex Mirror
Here is an experiment we did in the science laboratory on whether the angle of reflection is equal to the angle of incidence using a plane mirror and a concave mirror. the light ray came from a light box as shown below:
Here is the results of our experiment both with the plane mirror and with the concave mirror:
Plane mirror
Concave mirror
Colours:
In 1665, Issac Newton discovered that by placing a triangular prism in the path of a beam of white light, the light was split into seven bands of different colours, red, orange, yellow, green, blue, indigo, violet. This is called the visible spectrum. The process of separating white light into its constituent coloured lights is known as the dispersion of light.
This splitting of light can be attributed to the wavelength of each different colour. Red light has the longest wavelength and thus, it bends the least while violet light has the shortest wavelength and bends the most.
In order to recombine white light, another identical prism can be placed and inverted in front of the first prism. All the seven bands of coloured light will recombine into white light once again.
The 3 primary colours of light are: red, green and blue. Here are a few combinations of the primary colours of light to form other colours.
Green + Red = Yellow
Blue + Green = Cyan
Red + Blue = Magenta
When white light is shone at light filters, the filter absorbs one or more colours of light and the rest is transmitted. However, when a beam of single coloured light is shone at a filter that is made to transmit light of another colour, the resulting colour of light transmitted is black.
Example: when red light is shone on a green filter, black light is transmitted
On the other hand, when coloured light is shone on an opaque object, the opaque object reflects certain colours of light only. The rest is absorbed by the object. For example, a blue chair absorbs red and green light and reflects the blue light into our eyes such that the chair appears blue. Similar to light filters, when light is shone on a coloured object that does not reflect any colour of light found in the beam of light, the object appears black.
My Reflections: Reflection, refraction and the colours of light are pretty basic topics. Once I had the foundations covered, it became easier and easier. Lenses, on the other hand, was a totally different topic. It required massive amounts of brain power to memorize all 6 cases and 1 case for the convex and concave lenses. This was another of the killer topics for me as I had limited interest in this field, found it hard to memorize stuff just using raw brain power and I had to memorize many other things at the same time. As a result, I ended up spending more and more time on lenses and less time on other topics. However, to this date, I am still unable to memorize all of the conditions, images and many others for the two different types of lenses. Will keep trying! :D
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