Dispersion Of Light And Natural Optical Phenomena

Dispersion is the natural phenomenon by which white light is split into seven constituent colours.

Fig. Dispersion of light by prism

Dispersion is the natural phenomenon by which white light is split into seven constituent colours.

Fig. Dispersion of light by prism

In a transparent medium (like prism), the velocity of violet light is minimum which leads to maximum deviation.

Red. Green, Blue are the primary colours because every other colour can be obtained by recombining these colours.

Magenta, Yellow and Cyan are the primary pigments.

The light rays after being refracted from the eye lens falls on the retina where the image is formed.

Magenta is the complementary colour of blue.

Magenta colour is obtained by mixing the blue and red colours.

The focal length of an eye lens is changed due to the action of ciliary muscles.

Fig. Function of ciliary muscles

When the white light is incident on the blue pigment Violet, Green, Blue are reflected while the other colours are absorbed. This is known as colour subtraction.

Presbyopia is the decrease in accommodation power with ageing which can be corrected by bifocal lenses.

Absorption does not play a role in the formation of rainbow.

Total internal reflection occurs when light rays travels from a medium of greater refractive index to a medium of lower refractive index and the angle of incidence must be greater than the critical angle.

Fig. Total internal reflection

For a person suffering from defect of near sightedness the light rays meet ahead of the retina which is corrected by using a concave lens.

Fig. Image correction in myopia

The phenomenon responsible for the twinkling of the stars is atmospheric refraction which occurs due to the dense air in the earth’s atmosphere. Due to mobility and the change in temperature of the air the refractive index keeps on changing continuously and randomly. So, the apparent position of the star is not steady and it appears to be twinkling.

Scattering is the natural phenomenon where the light rays are directed in different directions when it interacts with small particles.

The time difference between actual sunset and apparent sunset is 2 minutes.

The particles present in the atmosphere have the same size as the wavelength of blue light, so the blue light is scattered more by the fine atmospheric particles.

Red light has the longest wavelength so it travels the fastest.

Dispersion is the natural phenomenon by which white light is split into seven constituent colours. The band of the seven colours obtained from the splitting of white light is called spectrum. All the constituents of white light travelling in vacuum they have the same velocity but when they pass through a transparent medium their velocity changes and they tend to deviate.

Fig. Dispersion of white light by a prism

The colour spectrum obtained from the dispersion of white light are:

Ø V – Violet

Ø I – Indigo

Ø B – Blue

Ø G – Green

Ø Y – Yellow

Ø O – Orange

Ø R - Red

The primary colours are Red, Green and Blue.

The colours obtained from their mixture are given by the table:

The coloured substances that are used as paint are known as pigments. The paints used for painting infrastructures and drawing are well-known examples of pigments.

The primary pigments are cyan, magenta and yellow.

The muscular structures which hold the eye lens in its position are known as ciliary muscles, they adjust the focal length of the lens by adjusting its thickness. The position where the image is formed due to refraction from the eye lens is called retina, the light-sensitive cells in the retina generate electric signals which are then transferred to the lens to interpret.

The ability of the eye lens to adjust its focal length as per requirement is called accommodation power of lens.

The ciliary muscles modify the curvature of the lens which in turn modifies the focal length of the eye lens. In the relaxed state of the ciliary muscles, the lens is thin and the focal length is more which enables us to see distant objects clearly. When the ciliary muscles contracts, the focal length decreases due to increased curvature of the lens and we are able to see near objects clearly.

The least distance of distinct vision for a young adult with normal vision is 25cm.

The density of earth’s atmosphere is not same everywhere, the layer near the earth’s surface has more density than the air at higher altitude. So, the refractive index keeps on changing continuously and randomly. Hence, the apparent position of the objects in distant outer space also keeps on changing. This phenomenon is known as atmospheric refraction.

The phenomenon such as twinkling of stars, early sunrise and delayed sunset occurs due to this effect.

Actual sunrise occurs when sun appears at the horizon.

During sunrise even the sun is lower than the horizon, the sun rays due to atmospheric refraction reaches our eyes and we experience sunrise before two minutes.

Fig. Apparent position of sun during sunrise.

Scattering is the natural phenomenon where the light rays are directed in different directions when it interacts with small particles.

Scattering depends on the size of the particles, if the particles are very small then it will scatter light of smaller wavelength (blue light) and if the particle size is slightly bigger then it will scatter light of greater wavelength (red light). If the particles are much bigger in size then they will scatter white light.

The red colour light is scattered least by the smoke or fog, so it can be seen from a long distance. Hence, the red light is used in danger signals.

During sunrise and sunset the sun rays (white light) have to travel a greater distance compared to any time of the day. While travelling the blue light of the sunrays gets scattered, only the red light reaches our eyes. Hence, at sunrise and sunset the sun appears red.

When the light rays travel from an optically denser medium to an optically rarer medium, the light rays are completely reflected back to the denser medium if the angle of incidence is greater than the critical angle. This phenomenon is known as total internal reflection.

Sparkling of diamond and working of optical fibre are both governed by this principle.

Fig. Total internal reflection in an optical fibre

Looming is a kind of mirage observed in very cold region in which the distant objects appear to be hanging midway in the air, virtual and erect. This is caused due to total internal reflection in downwards direction caused by atmospheric refraction when the warmer (optically rarer) air remains above the colder (optically denser) air in the atmosphere.

Fig. Looming

The primary colours red, blue and green can be combined in various combinations and proportions to get different colours. This phenomenon is known as superposition of primary colours.

The method of producing a wide range of colours by the appropriate mixture of this primary colours is known as additive mixture method.

Fig. Mixing of primary colours of light

The function of the main parts of an eye is:

1. Iris: The muscular diaphragm behind the cornea that controls the amount of light that enters the eye.

2. Pupil: The aperture of an eye behind the cornea whose size can be controlled by the iris.

3. Ciliary muscles: The muscular structures that hold the eye lens in its position and also control their aperture.

4. Retina: It is a light-sensitive tissue where the image is formed after refraction.

Fig. Structure of eye

The rainbow in a natural spectrum visible in the sky after rainshower. It is formed by the dispersion of light by the tiny droplets present in the atmosphere, the water droplets act like prism. It disperses the incident light first, and then reflects it internally (not necessarily total internal reflection) and finally refract it again while it comes out of raindrop.

Fig. Formation of rainbow

The phenomenon responsible for the twinkling of the stars is atmospheric refraction which occurs due to the dense air in the earth’s atmosphere. Due to mobility and the change in temperature of the air the refractive index keeps on changing continuously and randomly. So, the apparent position of the star is not steady and it appears to be twinkling.

Tyndall effect is the phenomenon in which the path of the light ray is visible when it travels in a colloidal solution.

Examples: light rays visible in dense forest or a smoke-filled room.

When the sun rays travel through the atmosphere it interacts with small particles in the air that leads to scattering of the sun rays and blue light is scattered. This makes the sky to appear blue in colour blue.

Dispersion is the natural phenomenon by which white light is split into seven constituent colours. The band of the seven colours obtained from the splitting of white light is called spectrum. All the constituents of white light travels in vacuum they have the same velocity but when they pass through a transparent medium their velocity changes and they tend to deviate.

Fig. Dispersion of white light by a prism

The colour spectrum obtained from the dispersion of white light are:

Ø V – Violet

Ø I – Indigo

Ø B – Blue

Ø G – Green

Ø Y – Yellow

Ø O – Orange

Ø R - Red

Mirage is a type of optical illusion that is generally seen in deserts and in very cold regions.

When the light rays coming from tall objects pass through the air of gradually decreasing refractive index( as the air gradually becomes hot), the light rays bend gradually away from the normal and their angle of refraction increases gradually so that they enter observer’s eye after total internal reflection occurs. Hence, the image formed in a mirage is virtual and upside down.

Fig. Diagram of a mirage

Defects of human eye is simply the inability of the ciliary muscles to change the thickness of the eye lens. They can be classified into three types:

1. Near-sightedness: Myopia or near sightedness is an eye defect in which a person can see clearly only when the objects are near. This happens when the lens becomes too thick and the light rays undergo greater refraction which makes the light rays to converge before the retina, making the eyesight blurry.

Using a concave lens can be helpful as it concentrates the light rays on the retina. The below image explains the process

Fig. Correction of myopia

2. Hypermetropia: Hypermetropia or far-sightedness is an eye defect there is difficulty with near vision but far objects can be seen easily. The image is focused behind the retina rather than upon it. This occurs when the eyeball is too short or the refractive power of the lens is too weak. Hypermetropia can be corrected by wearing glasses/contacts that contain convex lenses.

Fig. Correction of Hypermetropia

3. Presbyopia: Presbyopia is the decrease in accommodation power with ageing which can be corrected by bifocal lenses. This defect is caused due to weakening of ciliary muscles and loss of elasticity of the eye lens. This can be treated by using bifocal lens.