Content Curator | Updated On - Apr 9, 2024
Ray optics and optical instruments provide an interesting look into the world of light, showing its behavior and allowing us to change it for a variety of reasons.
- Optics is a branch of physics that deals with the study of nature, production, and propagation of light.
- Optics is divided into two categories: 1. Ray optics or Geometrical optics 2. Wave optics or Physical optics
- Ray Optics is a branch of physics that studies the behavior and properties of light.
- In Ray Optics, the light travels in a straight line and states that there is an image for each and every object.
- The optical devices out there that have been made utilizing the reflecting and refracting properties of mirrors, lenses, and prisms are the optical instruments.
Key Terms: Ray Optics, Optical Instruments, Reflection, Refraction, Dispersion, Diffraction, Polarization, Scattering, Transverse Wave, Laws of reflection. Snell’s law, Total internal reflection
What is Ray Optics?
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Ray Optics explains the propagation of light in terms of rays.
- It is also known as Geometrical optics.
- It is based on the rectilinear propagation of light.
- It deals with the reflection of light, refraction of light, and formation of images by mirrors, prisms, and lenses.
- It deals with the behavior of light.
- Light is usually considered a ray that travels in a straight path.
- Optics is a branch of physics that explains the behavior of visible light, ultraviolet, and infrared light.
- The path traveled by light is called Ray. In addition, a bunch of light rays is known as a Beam of light.
Ray Optics
Characteristics of Light
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There are several characteristics of light. Some of them include:
Light Always Travels in a Rectilinear Propagation
Light generally chooses the straight path to travel. That means if the path has curves or bents, light rays cannot continue their propagation. That means light chooses a short way to propagate.
Rectilinear Propagation
Light Needs No Form of Medium
Light can travel in any medium. One of the major characteristics of Light is that it can travel through vaccum very easily as well.
Speed
There is no other source of energy, which can travel as fast as light. The speed of light travelling via vaccum is a primary physical constant.
| Speed of Light (c) = 3 × 108 m/s |
Usually, the wavelength of light changes when it passes from one medium to another.
| Ques. What are some important properties and types of light? Ans. Some of the major properties and types of light include:
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What is Reflection?
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When a light ray is propagated, if that light ray hits any surface and travels back at the same angle, it is called reflection.
- The surface that reflects the light is known as the reflecting surface.
- Reflection of light obeys certain laws known as the Laws of Reflection.
Laws of Reflection
According to the laws of reflection
- The angle of incidence is equal to the angle of reflection, i.e. ∠i = ∠r
- The incident ray, the reflected ray, and the normal to the reflecting surface lie in the same plane called the plane of incidence.
Reflection of light
In the above figure
- The angle made by the incident ray with respect to a normal line or surface is called the Angle of incidence (i)
- And, the angle made by the reflection with respect to the normal line or surface line is called the Angle of reflection (r)
According to Snell’s Law of Reflection, when the light is focused on a surface AB, which is a regularly polished surface that means no curves the light will be reflected back with the same intensity.
Let us consider drawing a line at the point of contact with the surface, which is called a normal line. In such a situation:
- The angle of incidence is equal to the angle of reflection
- The incident ray, normal, and reflected ray lie on the same plane
- In the case of plane mirrors, the reflected image will be slightly inverted which is called a laterally inverted image.
Reflection of Light on Spherical Surfaces or Spherical Mirror
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When a spherical mirror is cut horizontally or vertically, it will be divided into two curves. Those are called curved surfaces, which have a reflecting surface on one side and an objective surface on another side.
Reflection on spherical surface
There are two curved surfaces, namely concave and convex mirrors.
- Concave Mirror: When the reflecting surface is outwards while bulged inwards, that kind of mirror is called a Concave mirror.
- Convex mirrors: When the mirror bulges outwards and the reflecting surface is inside, that kind of mirror is called Convex Mirrors. A convex mirror is the exact opposite of a concave mirror.
Focal length can be defined as the distance between the focus and the pole of the mirror.
- Focal length can be represented as f and is equal to half of the radius of mirror curvature.
- The focal length for the convex mirror is positive while the concave mirror's focal length is negative.
The mirror formula is represented as:
| 1/v + 1/u = 1/f |
Where,
- u = object distance,
- v = image distance,
- f = focal length
Magnification can be defined as the ratio of the size of the image to the size of the object. And, if magnification is positive then the image will be real and inverted. If magnification is negative, the image is virtual and erect.
What is Refraction?
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Refraction is the light that passes obliquely from one medium to another, causing the image to have a phenomenal change in its light path. Laws of Refraction include:
- The incident ray, normal line, and refracted image range lie in the same plane.
- The ratio of sine angle of incidence and sine angle of refraction stays constant for a given medium.
Refraction of light
Refractive index is defined as the ratio of the speed of light in a vacuum to the speed of light in the medium.
According to Snell’s law of Refraction,
- The ratio of the sine angle of incidence to the sine angle of refraction is called a refractive index with respect to the refracting medium to the incident medium.
sinisinr = n2/n1
- And, the absolute refractive index means the ratio of the speed of light in a medium to the speed of light in a vacuum.
n = c/v
The optically denser or rarer medium can be determined by using an absolute refractive index.
- If the absolute refractive index of medium 1 is higher than medium 2, then medium 1 is said to be an optically denser medium.
- And, if the refractive index of medium 1 is lower than medium 2 then medium 1 is said to be an optically rarer medium.
Principle of Reversibility:
- The ratio absolute refractive indexes of 2 mediums are called the Relative refractive index.
- If any object is placed underwater, it will appear as raised due to refraction of light when it is observed inclined and we can also observe a shift in the object’s position.
The phenomenon of light, which reflects into a denser medium without refracting to the rarer medium is called Total Internal Reflection.
Scattering of Light Diagram
Conditions for Total Internal Reflection
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The conditions required for Total Internal reflection include:
- The propagation of light must be from an optically denser medium to an optically rarer medium.
- The angle of incidence must be higher than the critical angle of incidence.
Here, when the light ray enters into a rarer medium from a denser medium and the angle of incidence is higher than the critical angle of incidence, that light ray will be reflected back to the denser medium. This is called Total Internal Reflection. Examples include: Prisms, Optical Fiber, Diamonds, Mirage, etc.
Refraction Through Spherical Surfaces
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Len’s Maker’s formula for the thin lens will be 1f = (n2 – n1n1)(1R1 – 1R2)
Refraction through Spherical Surfaces
R1 is positive and R2 is negative for convex lens whereas R1 is negative and R2 is positive for the concave lens. A combination of the lens is useful for:
- Increasing the magnification of an image
- Reducing the defects of an image caused by a single image, which in turn increases the sharpness.
- Giving the erected image with respect to the object
- Increasing the field view.
When the light is passed through a prism, the light undergoes refraction. As the angle of incidence increases, the angle of deviation decreases reaching a minimum value. That minimum value is called the angle of deviation.
Also Read:
Previous Year Questions
- A and B are two parallel-sided transparent slabs of refractive indices … [WBJEE 2016]
- A ray of light passes through four transparent media with refractive indices … [JEE Advanced 2001]
- A transparent slab of thickness dd has a refractive index n(z) … [JEE Advanced 2016]
- The angle of minimum deviation for an incident light ray on an equilateral prism … [KCET 2009]
- In refraction, light waves are bent on passing from one medium … [KCET 2021]
- Two plane wavefronts of light, one incident on a thin convex lens … [KEAM]
- For a crystal, the angle of diffraction … [BITSAT 2008]
- When light is incident on a diffraction grating the zero-order … [KCET 2004]
- The width of the diffraction band varies … [KCET 2006]
- For a transparent medium relative permeability and permittivity … [NEET 2019]
- A transparent cube of 0.21m edges contains a small air bubble … [KCET 2006]
- Reflection of the point … [KCET 2017]
- For a normal eye, the cornea of the eye provides a converging power … [NEET 2013]
- Focal length of a convex lens of refractive index … [NEET 1988]
- Find the value of the angle of emergence from the prism … [NEET 2021]
- Assume that light of wavelength 600nm is coming from a star … [NEET 2020]
- An object is placed at a distance of 40 cm … [NEET 2018]
Important Topics for JEE MainAs per JEE Main;2024 Session 1, important topics included in the chapter Ray Optics and Optical Instruments are as follows:
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Things to Remember
- Optics is a branch of physics where we study the behavior and properties of light.
- In Ray Optics, which is also regarded as Geometrical Optics, the light travels in a straight line and states that there is an image for each and every object.
- The optical devices out there that have been made utilizing the reflecting and refracting properties of mirrors, lenses, and prisms are the optical instruments.
- When the light is passed obliquely from one medium to another, then the image will have a phenomenal change in its light path, this is called Refraction.
- When the light ray enters into a rarer medium from a denser medium and the angle of incidence is higher than the critical angle of incidence, that light ray will be reflected back to the denser medium. This is called Total Internal Reflection.
- When the light is passed through a prism, the light undergoes refraction.
Sample Questions
Ques. A concave lens with a refractive index of 1.5 is immersed in a medium with a refractive index of 1.65. Determine the nature of the lens. (2 Marks)
Ans. If a concave lens with a refractive index of 1.5 is immersed in a medium with a refractive index of 1.65, the nature of the lens will be converging.
Ques. When an object is placed between f and 2f of a concave mirror, would the image formed by
(i) virtual or real and
(ii) diminished or magnified? (2 Marks)
Ans. (i) When an object is placed between f and 2f of a concave mirror, the image formed is real.
(ii) Secondly, the image formed would be magnified.
Ques. If the incident violet light is replaced by a red light, how does the angle of minimum deviation of the glass prism vary? Give a reason. (2 Marks)
Ans. When a violet light is replaced by a red light, the angle of minimum deviation decreases as the refractive index for the violet light is more than that for red light.
Ques. The speed of light decreases when light travels from a rarer to a denser medium. Does the reduction in speed imply a reduction in energy? (2 Marks)
Ans. When light travels from a rarer to a denser medium, the reduction in speed does not imply a reduction in energy.
Ques. (a) Draw a ray diagram for a convex minor that shows the image formation of an object that is placed anywhere in front of the minor.
(b) By using this ray diagram, obtain the expression for its linear magnification. (3 Marks)
Ans. (a)
Ques. Draw a schematic arrangement of a reflecting telescope (Cassegrain) that shows how rays coming from a distant object are received at the eye-piece. What are its two advantages over a refracting telescope? (3 Marks)
Ans. A reflecting telescope is a telescope with mirror objectives which is also known as Cassegrain or Newtonian telescope. The ray diagram of the reflecting type telescope is shown in the figure below,
Its two advantages over a refracting telescope are as follows:
- The image formed is free from chromatic aberration as the reflection mirror has mirror objectives.
- As the spherical mirrors are parabolic mirrors, free from spherical aberrations, they tend to produce a very sharp and distinct image.
Ques. (a) Obtain the expression by drawing a suitable ray diagram for the refractive index of the triangular glass prism in terms of the angle of minimum deviation D and the angle of prism A. Draw a plot that shows the variation of the angle of deviation with the angle of incidence. (b) Calculate the value of the angle of incidence when a ray of light is incident on one face of an equilateral glass prism and produces the emergent ray which just grazes along the adjacent face. The refractive index of the prism is √2. (5 Marks)
Ans. The figure displays the passage of light through a triangular prism ABC. The angles of incidence and refraction at the first face AB are i and rv whereas the angle of incidence from glass to air at the second face AC is r2 and the angle of refraction or emergence e. The angle between the emergent ray RS and the direction of the incident ray PQ is called the angle of deviation.
In the quadrilateral AQNR, two of the angles at the vertices Q and R are right angles. Thus, the sum of the other angles of the quadrilateral is 180 degrees.
From the given triangle,
By comparing these two equations we get,
r1 +r2 = A
The total deviation 8 is the sum of the deviations at the two faces.
… (i)
The plot between the angle of deviation and that of incidence is shown in the figure. Generally, any given value of deviation except for i = e corresponds to two values, i and e. This is expected from the symmetry of i and e in equation 1, i.e., deviation remains the same if i and e are interchanged. Physically it is linked with the fact that the path of the ray in the diagram can be tracked back resulting in the same angle of deviation. At a minimum deviation dm, the refracted ray inside the prism becomes parallel to its base, we have,
When the angle of incidence and that of emergence are equal i.e.,
(b) Give, μ = √2, then ∠i = ?
As the emergent ray just grazes along the adjacent face of an equilateral glass prism,
Ques. Two monochromatic rays of light are incident normally on the face AB of an isosceles right-angled prism ABC. The refractive indices of the glass prism for the two rays 1 and 2 are 1.35 and 1.45 respectively. Trace the path of these rays entering through the prism. (5 Marks)
Ans. (i) For the ray 1,
As the angle of incidence 45° is less than the critical angle 48°, the ray will be refracted.
(ii) For ray 2,
As the angle of incidence here is more than the critical angle, the ray will be totally internally reflected.
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