PHYSICS: COLOURS OF LIGHT

Colours of Light

The Component of White Light

Explain the component of white light

There are two types of colour of light

1. Primary colour of light
2. Secondary colour of light

Primary colour of light

These are basic (fundamental ) Colour of light to which the eye is most sensitive.Primary Colour of light Include the following

1. Red
2. Green
3. Blue

Secondary colours of light

These are colour of light obtained after mixing primary colours of light

Colour mixing by Addition

This is the process of combining primary colours of light without loss any colour to form secondary colours of light.

	Primary color	Secondary color
	Red + Blue	Magenta
	Red + Green	Yellow
	Blue + Green	Cyan

Colours of White Light

Recombine colours of white light

When all white light ( Red , Blue and Green)Combineforms WHITE LIGHT.

Complementary colours of light: These are the colours which produce white light when combined.

• Red + Blue+ Green - White light
• Red + Cyan - White light
• Blue + Yellow - White light
• Green + Magenta - White light

The Appearances of Coloured Object under White Light

Explain the appearances of coloured object under white light

There are two types of coloured paints ( pigments) which Include the following

1. Primary coloured pigment (paints)
2. Secondary coloured pigment (paints)

Primary, Secondary and Complementary Colours of Light

Identify primary, secondary and complementary colours of light

Primary Coloured pigments

These are basic coloured pigments which form secondary coloured pigment when combined.

The primary coloured pigments include:Yellow, Cyan and Magenta

Secondary colour pigments

These are coloured pigments which are formed when two primary colours combine, whichis always accompanied with the removal of other colours.

Difference between Additive and Subtractive Combination of Colours

Distinguish between additive and subtractive combination of colours

Colour Mixing by Substration

Is the process of mixing two primary coloured paints ( pigments) to form secondary colour white.

Example 3

• Magenta + Cyan
• Magenta = ( Blue) + ( Red)
• Cyan = (Blue) + (green)

The colour which is common to Blue will appear while red and green disappear.

Magenta + Cyan = Blue

Example 4

• Magenta + yellow
• Magenta = (Blue) + (Red)
• Yellow = (Green) + (Red)

The colour which is common to both red will appear while blue and green will disappear.

Hence

Magenta + Yellow = Red

Example 5

• Cyan + yellow
• Cyan = (Blue) + (Green)
• Yellow = (Red) + (Green)

The colour which is common to both green will appearwhile Blue and Red will disappear

Hence

Cyan + Yellow = Green

Read More

PHYSICS:REFRACTION OF LIGHT BY REGULAR PRISM

Refraction of Light by Rectangular Prism

The Passage of Light through a Triangular Prism

Trace the passage of light through a triangular prism

Deviation of light in a prism is the changing in direction of the incident ray when it enters/hits a triangular glass prism.

Where i

is the angle of incidence

s is the angle of deviation

The minimum angle of deviation ( qm)

In order to determine the minimum angle deviating (Qm) then we must set triangular Glass prism as follows.

The Dispersion of White Light

Demonstrate the dispersion of white light

Dispersion of light is the splitting up of light beam (white light) into its seven components of colour by a prism.

Spectrum is the patch or band of colours which comprise / constitute seven component of white light.

Pure section is the patch or band of colours in which the colours are clearly separated.

In order to produce pure spectrum then we must use two converging lenses (convex lenses).

When colours of spectrum are combined, they form white light.

In order to combine colours of the spectrum, weneed two triangular glass prisms and one lens.

Impure spectrum:the band/patch of colours which overlap and are not seen clearly.

The rainbow:a bow-shaped spectrum of seven colours of white light formed when white light undergoes dispersion within the rain drops because water is denser than air, so has a large refractive index.

Activity 1

A rainbow can be demonstrated as follows:

Spray some water into the air in a direction opposite to that of the sun.

Look at the water shower while you face away from the sun. You will see the colour of the spectrum of white light in the falling drops of water. The spectrum so formed hasthe shape like a bow. So it is called rainbow.

There are two main types of rainbow:

1. Primary rainbow
2. Secondary rainbow

Primary rainbow

This is formed when light undergoes one or single total internal reflection in the water droplets. In this type of the rainbow the violet colour is on the inside of the bow while the red colour is on the outside.

The Angles of Deviation and Minimum Deviation

Determine the angles of deviation and minimum deviation

Finding the refractive index (n) of glass by using the deviation of light in a prism:

Refracting angle of prism is A

Snell’s law

S in i/Sin r = N

Sin i= n sin r

Sin e’ = nsin i

From Geometry of figure

I = A- r

The total angle of deviation (s) is the angle between the direction orf the incident ray and the emergent ray.

Again from the Geometry Q is given by:

S= i+r’- A

When the deviation is a minimum (Sm) the passage of light through the prism will be symmetrical so:

I = r ‘and r = I’

This means that;

A + Smin = 2i = 2r’

Therefore;

Refractive index, n = Sin (A + Smin)/2

Sin (A/2)

Where

A = Apex angle ( angle of prism)

Smin – The angle of minimum deviation

A Simple Prism Binocular

Construct a simple prism binocular

Simple prism binocular

Read More

PHYSICS:REFRACTION OF LIGHT

Refraction of Light

The Concept of Refraction of Light

Explain the concept of refraction of light

Refraction of light refers tothe bending of light as it passes through two different medium because the speed of light tends to change when travelling from one medium to another.

Figure showing refraction of light as it passes from air to glass.

The Angle of Incidence and Angle of Refraction

Measure the angle of incidence and angle of refraction

The angle of incidence (i)is the angle between the incident ray of light and the normal at the point of incidence.

The angle of Refraction (r)is the angle between the refracted ray and the normal at the point of incidence.

The Laws of Refraction

State the laws of refraction

First law of refraction

The First Law of refraction states that "the incident ray, the refracted ray and the normal at the point of incident are located in the same plane.”

Second law of refraction

Second Law of refraction states that “when a light ray passes from one medium into another medium, the angle of incidence (i) and corresponding angle of refraction( r) are such that the ratio of sine of the angle of incidence to the sine of the angle of refraction (sini/sinr) is a constant value called the refractive index."

Note: The Second Law of Refraction is called Snell's Law in honour of a Dutch scientist named Snell (1591 – 1626) who first described it.

The Refraction Index of a Material

Determine the refraction index of a material

Refractive index (n) is the ratio of the sine of the angle of incidence to the sine of the angle of refraction.

n = Sini/Sinr OR

Refractive index (n) is the ratio of the velocity of light in air to the velocity of light in glass.

n = Velocity of light in air (Va)/Velocity of light in glass (Vg)

Or

Refractive index, n is the constant number which expresses how many times or to what extent a light ray bends when passing through different medium.

Absolute refractive index (n_a) is the refractive index between vacuum or air and any other medium.

The refractive indices between air and some common media is given below:

Medium	Refractive index (n)
Diamond	2.417
Ethanol	1.360
Glass (Crown)	1.520
Quartz	1.553
Water (at 20ºC0	1.333
Air (at stp)	1.00029

Example 2

The refractive index for light passing from air to water is equal to 1.333 find the refractive index for light travelling from water to air.

Data given:

Refractive index an_w of air to water = 1.333

Required: To find refractive index from water to air

Since

anw = 1.333

wna = (1/anw)

= (i/1.333)

: wna = 0.75

Real and Apparent Depth

Real depthis the actual height measured without taking account any refraction of light

Apparent depth is the virtual height measured when viewed by observer.

The Concept of Critical Angle and Total Internal Reflection of Light

Explain the concept of critical angle and total internal reflection of light

Critical angle

Critical angleis the angle of incidence (i) for which the angle of refraction (r) is equal to 90º . It is obtained when light rays moves from a dense medium to a less dense medium.

For refractive index

n=Sini/sinr

But i= Critical angle, C

r = 90º

Thus n= sinC/sin 90º

n=SinC/1

n = Sin C

c= Sin -1 (n)

Total Internal Refraction

This occurs when a light ray from a less dense medium is reflected into the denser medium at the boundary separating the two media.

Conditions for total internal reflection to occur include the following:

1. Light must be travelling from a more dense to less dense medium.
2. Light must incident at the boundary at an angle greater than the critical angle (C).

Optical fibres

These are very thin tubes of plastic or glass and because they are so thin they can bend without breaking, so they can carry light around the corners.

Uses of optical fibres

• Used in telecommunications to carry telephone calls over vast distance, without loss of intensity and without interference.
• Used in endoscope to view inside a patient body for example inside stomach. Light is carried into the stomach through a bunch of fibres and is reflected into small camera, which then displays a picture on a screen.

The Occurance of Mirage

Explain the occurrence of mirage

This is the phenomenon inwhich an object appears to be at an incorrect position due to the bending of light rays from the object.

Mirages occur during hot days.

Read More