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Geometric optics is an optical model that tells about the propagation of light in terms of rays. In geometric optics, the rays of light are vital to approximately find the path along which light travels in various conditions.
There are various theories about geometric optics, which are:
- Light rays travel in a straight line path when it travels through a transparent medium.
- Light rays change the path at the interface of two with variant refractive indexes.
- Light rays will be absorbed, reflected, and refracted.
Light Geometric Optics is a brilliant method to describe the behavior of light as its wavelength is comparatively very small. Geometric Optics is mainly made use of in the process to describe the geometric aspects of imaging such as optical aberrations.
| Table of Content |
KeyTerms: Light, Waves, Dispersion, Light rays, VIBGYOR, Optics, Laws of Light, Prism, Reflection, Refraction, Ray optics, Photon, optical aberrations
What is Geometric Optics?
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Geometrical optics is also known as ray optics. It is a type of optics that talks about the rectilinear propagation of light in terms of rays.
- Light can work in many ways as it is dual-natured.
- It is wave natured and also has a particle nature in the form of a photon.
- Light moves in a straight line & this particular property of light is known as rectilinear propagation of light.
- Geometrical optics generally deals with the rectilinear propagation of light and other properties such as reflection, refraction, and polarization.
- For ease, optics is divided into two sections based on the various particle sizes that interact with light.
When light contacts an object that has a wavelength much bigger than the wavelength of light, then the behavior of light is similar to a ray, and it doesn't project any wave nature. This type of optics is known as “geometric optics”.
Refraction of Light Through a Prism Experiment
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Main Objective
The main objective refraction of light through prism are as follows:
- To study the angle of deviation ‘d’, and to find the angle of minimum deviation ‘D’ from the i-d curve for an angle of incidence ‘i’.
- To find out the refractive index of the material of the prism, make use of angle A (angle of the prism) and angle D.
Theory
Prism
A prism is an optical device made with polished flat surfaces which refract light.
- A prism that has a triangular base and rectangular sides is known as a triangular prism.
- A prism is generally made up of substances such as glass, plastic, and fluorite.
- A prism is made to split white light into its various components.
Read more:
| Relevant Concepts | ||
|---|---|---|
| Refraction of light | Brewster’s law | Critical angle formula |
| Plane mirror | Colloids | Dispersion forces |
How does a Prism Work?
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When there is a propagation of light from one place to another, it gets refracted by an angle. The angle of refraction of light will depend on the angle of incidence,
This is known as Snell's law and is given by:
| n = sin i/sin r |
Where,
i = the angle of incidence
n = the refractive index of the material of the prism
r = the angle of refraction.
- Various wavelengths of light refract variety, and the refractive index differs with the wavelength of light used.
- Light of various colors (or wavelengths) refracts variety and moves from the prism with different angles. This is known as dispersion.
This property shows that light can be used to separate a single beam of light into various colors.
The relation between Refractive Index 'n', Angle of Minimum Deviation 'D', and Angle of Prism ‘A’ is below:
Let us consider a triangular prism.
A triangular prism
The two refractive surfaces of the prism are considered ABFE and ACDE, respectively. The angle between them, angle ‘A’ is known as the angle of the prism.
A ray of light passing through a prism will undergo two refractions while passing through the prism.
Let us consider KL to be a monochromatic beam of light that falls on the AB of the prism. It will get refracted passing through the path LM. At point M it refracts again and moves out along MN.
This angle of deviation can be described as the angle where the emergent ray moves away from the extending direction of the incident ray.
The refractive index 'n' of the material of the prism is obtained by using the formula.
| N = (sin (A+D)/2)/sin A/2 |
The Laws of Reflection in Geometric Optics
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The passage of the ray of light that is rebounded by a smooth mirror is calculated depending on the laws of reflection. But basically, one will need to understand the basic terms.
Reflected Ray
Light moving toward a mirror is known as an incident ray and the beam of light that is reflected and moved away by the mirror is known as a reflected ray.
Normal
At the point of reflection, the perpendicular which is drawn to the middle is known as normal.
The angle of incidence
The angle of incidence is the angle formed by the incident ray and the normal, while the angle of reflection is the angle formed by the reflected ray and the normal.
According to the laws of light reflection, the angle of incidence is equivalent to the angle of reflection.
Law of reflection
Law of Refraction
Snell's Law is another name for the law of refraction. According to snell’s law, “the ratio between the sine of the angle of incidence and sine of the angle of refraction in two separate mediums wherein the light travels is equal to the ratio of the medium's phase velocities.” It is also equal to the reciprocal of the ratios of the indices of refraction.
Snell’s law
Refractive Index of the Medium in Geometric Optics
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The refractive index is represented by 'n' and is known as a dimensionless number which tells that the light or any other radiation moves through a medium. Which is mathematically depicted as:
| n = c/v |
where,
‘v’ represents the phase velocity of light in a medium, and
‘c’ represents the speed of light in a vacuum.
Refraction through a Glass Prism
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The ray of light consists of various colors and is also known as white light. When the white light moves through a prism, it is divided into various colors it is made up of, which are violet, indigo, blue, green, yellow, orange, and red. This process is also known as dispersion which occurs due to a difference in the wavelengths of each color. Because of these variant wavelengths, the dispersed ray from the prism refracts into various angles and thus displaying all the colors that are in the white light. This color is always in the configuration which is termed VIBGYOR.
Refraction of white light
Things to Remember
- When light contacts an object that has a wavelength much bigger than the wavelength of light, then the behavior of light is similar to a ray, and it doesn't project any wave nature.
- Snell's Law is also known as the law of refraction.
- At the point of reflection, the perpendicular which is drawn to the middle is known as normal.
- A prism is made to split white light into its various components.
- According to the laws of light reflection, the angle of incidence is equivalent to the angle of reflection.
- Dispersion is the light of various colors (or wavelengths) refracts variety and moves from the prism with different angles.
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Sample Questions
Ques. What is the relative refractive index? (1 mark)
Ans. The relative refractive index can be described as the ratio of the speed of light in one medium to that in another medium.
Ques. Explain Dispersion through a Glass Prism. (3 marks)
Ans. Dispersion is the phenomenon in which white light is split into vibgyor colors or wavelengths.
- The various colors that disperse from the prism are always in an order called 'VIBGYOR. Namely;
V-Violet
1- Indigo
B-Blue
G-Green
Y - Yellow
0-Orange
R-Red
- The band of the colored vibgyor of light is called the spectrum of light
Ques. How is Geometric Optics important? (2 marks)
Ans. With the help of geometric optics one can better understand the situations such as reflection and refraction by geometric optics. The use of objects such as mirrors, lenses, prism, and fibers is all dependent on geometric optics.
Ques. What does Ray Optics mean? (1 mark)
Ans. Ray optics is the variant name for geometric optics. Light rays always travel in a rectilinear path When light travels and moves through space, its amplitude oscillates (like other waves).
Ques. What is the difference between Physical Optics & Geometric Optics? (1 mark)
Ans. Geometric optics observe the optical properties of light such as refraction and reflection, whereas, physical optics deal with the study of the principle of wave nature of light.
Ques. What does VIBGYOR mean? (1 mark)
Ans. VIBGYOR is described as Violet, Indigo, Blue, Green, Yellow, Orange, and Red which is the order of light rays obtained when dispersed.
Ques. State Snell’s law. (4 marks)
Ans. According to snell’s law, “the ratio between the sine of the angle of incidence and sine of the angle of refraction in two separate mediums wherein the light travels is equal to the ratio of the medium's phase velocities.”
n = sin i/sin r
where,
i = the angle of incidence
n = the refractive index of the material of the prism
r = the angle of refraction.
Ques. What is the formula to find the refractive index? (3 marks)
Ans. n = c/v
where,
‘v’ represents the phase velocity of light in a medium, and
‘c’ represents the speed of light in a vacuum.
Ques. What is refraction? (2 marks)
Ans. Bending of light when it passes from one medium to another is known as refraction. In the process of refraction, the light rays move away when they pass through from one medium to another medium.
Ques. Define these terms (3 marks)
(a) Reflected ray
(b) Normal
(c) The angle of incidence
Ans.
- Reflected Ray: Light moving toward a mirror is known as an incident ray and the beam of light that is reflected and moved away by the mirror is known as a reflected ray.
- Normal: At the point of reflection, the perpendicular which is drawn to the middle is known as normal.
- The angle of incidence: The angle of incidence is the angle formed by the incident ray and the normal, while the angle of reflection is the angle formed by the reflected ray and the normal.
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