Ray Optics and Optical Instruments Class 12 Notes: CBSE Physics Chapter 9

Class 12 CBSE Physics covers Ray Optics and Optical Instruments. This chapter holds significant weightage, contributing to a total of 18 marks in exams. Exploring phenomena like refraction, lenses, and optical instruments, it's crucial for understanding fundamental principles of Physics, making it a key focus for board exams and competitive entrances like JEE and NEET. Ray Optics and Optical Instruments Class 12 Notes have been subdivided into the following sub-sections to help students navigate through them easily:

Reflection of Light

This section from Chapter 9 Physics Ray Optics Class 12 Notes will help students to revise their understanding regarding the reflection of light. Accordingly, essential concepts such as deviation, laws of reflection have been discussed in detail. Reflection by a plane surface and a plane mirror has been explained in Ray Optics Class 12 Notes with the help of diagrams.

Spherical Mirrors

Students can strengthen their understanding of spherical mirrors after going through this section in the Class 12 Physics Chapter 9 Notes. A simplified explanation of the following terms have been provided under this section in Ray Optics Notes Class 12 PDF:

• Centre of curvature.

• Normal.

• Radius of curvature.

• Pole of mirror.

• Principal axis.

Concepts such as paraxial rays have also been discussed in this segment from Ray Optics And Optical Instruments Class 12 Notes with the help of ray diagrams. A step by step explanation has also been provided on mirror formula and magnification that will enable you to revise them at a glance before examination.

Sign Convention:

In Class 12 Physics Chapter 9, “Ray Optics and Optical Instruments,” the sign convention is very important for solving problems involving lenses and mirrors. Given below is an easy explanation.

For Lenses:

• Focal Length (f): Positive for convex lenses (converging) and negative for concave lenses (diverging).

• Object Distance (u): Always taken as negative because the object is placed to the left of the lens.

• Image Distance (v): Positive if the image is formed on the right side of the lens (real image) and negative if on the left side (virtual image).

For Mirrors:

• Focal Length (f): Positive for concave mirrors (converging) and negative for convex mirrors (diverging).

• Object Distance (u): Always taken as negative because the object is placed in front of the mirror.

• Image Distance (v): Positive if the image is real (formed on the same side as the reflected rays) and negative if virtual (formed behind the mirror).

These conventions help ensure consistency and accuracy in calculations involving optical systems.

Focal Length of Spherical Mirrors:

The focal length is the distance between the mirror’s surface and its focal point, where parallel rays of light converge or appear to diverge from. For spherical mirrors, which include both concave and convex mirrors, the focal length helps determine how they form images.

Concave Mirrors: These mirrors curve inward and have a focal point in front of the mirror. The focal length is positive, and the mirror converges light rays to a point.

Convex Mirrors: These mirrors curve outward and have a focal point behind the mirror. The focal length is negative, and the mirror diverges light rays, making them appear to spread out from the focal point.

Mirror Formula: The mirror formula connects the focal length of a mirror with the distances of the object and the image from the mirror. The formula is:

$\frac{1}{f} = \frac{1}{v} + \frac{1}{u}$

where f is the focal length, v is the image distance, and u is the object distance.

Refraction of Light

Within this section of Ray Optics And Optical Instruments Class 12 Notes, the following topics have been explained in a detailed manner so that students can comprehend them quickly:

• Reflective index.

• Law of Refraction or Snell’s law.

• Refraction through a curved surface.

• Total internal reflection.

• Double refraction from a plane surface.

• Single refraction from a plane surface.

• The relation between object distance and image distance refraction.

• Linear magnification for spherical refracting surfaces.

Furthermore, derivations of equations for each of the above terms are also provided in Class 12 Physics Chapter 9 Notes, which will help you to understand these concepts thoroughly.

Total Internal Reflection

When light moves from a denser medium (like glass) to a less dense medium (like air) and hits the boundary at an angle greater than a specific angle called the critical angle, it reflects completely back into the denser medium. This is known as Total Internal Reflection. It only happens if the angle at which the light hits the boundary is greater than this critical angle (i.e., the angle of incidence is greater than the critical angle).

Lens

After going through this section from Ray Optics Class 12 Notes, students will learn to distinguish between a thin lens and a standard lens. Some of the main terms related to lens have been explained in a simplified way to aid you in your revision process. These terms include:

• Centre of curvature.

• Radius of Curvature.

• Principal Axis.

• Optical centre.

• Principal foci.

• First principal focus F1.

• Second principal F2.

• Focal Length.

Apart from these terms, crucial concepts such as lens maker’s formula and lens formula, silvering of lens, a combination of lens, cutting of lens etc. have also been discussed in brief in Ray Optics Notes Class 12 PDF.

Power of lens:

• A lens with a positive power (convex lens) converges light and brings it to a point.

• A lens with a negative power (concave lens) diverges light and spreads it out.

• The greater the magnitude of the power, the stronger the lens’s ability to bend light.

Formula for Power of a Lens: P= $ \frac{1}{f}$,​ where P is the power of the lens in diopters, and f is the focal length of the lens in metres.

Combination of thin lenses in contact:

In Class 12 Physics Chapter 9, “Ray Optics and Optical Instruments,” the topic of "Combination of Thin Lenses in Contact" deals with how to handle multiple lenses placed together without any gap between them. When lenses are in contact, they work together to form a single optical system. 

To find the effective focal length of the combined lenses, you use the following formula:

$\frac{1}{f_{\text{effective}}} = \frac{1}{f_1} + \frac{1}{f_2} + \frac{1}{f_3} + \ldots$

where $f_{\text{effective}}$ is the focal length of the combination, and $f_1$,$ f_2$, $f_3$, $\ldots$ are the focal lengths of the individual lenses. This formula helps you determine how the lenses together will focus light and form images.

Prism

This section from Ray Optics And Optical Instruments Class 12 Notes covers the features of a Prism, its importance and various formulae applicable to it. Furthermore, students can also recapitulate their knowledge of multiple theories like dispersion and deviation of light by a prism, condition of no emergence, dispersive power which has been explained in a simple and straightforward way along with equations.

Optical Instruments

We use several optical instruments in our day to day lives which have been developed after applying the properties of reflection, refraction, dispersion etc. A brief discussion on the workings of these instruments has been given in Ray Optics And Optical Instruments Class 12 Notes. Some of the devices include:

Eye

Our eyes contain an array of interconnected nerve fibres and cells that can sense the intensity of light and colour. For instance, the retina contains nerve cells like rods and cones, which receives the light, converts it to electrical signals and sends it to the brain via the optic nerves. Furthermore, the ciliary muscles help in modifying the shape and focal length of the lens present in an eye.

Simple Microscope

A simple microscope which is also known as a magnifying glass is made of a converging lens which has a small focal length. Consequently, when it is held close to the eye magnified, an erect and virtual image is formed. On the other hand, a compound microscope has two converging lenses, an eyepiece with moderate focal length and large aperture, objective lens of small focal length and short aperture.

Magnifying Power (M): M =$ \frac{D}{F} \times \frac{L}{F_o}$​

Where D is the diameter of the objective lens, F is the focal length of the objective lens, L is the distance between the objective and eyepiece lenses, and $F_o$​ is the focal length of the eyepiece.

Telescope

This device is used to observe objects which are far away. However, a telescope has an objective lens of large aperture and considerable focal length and eye lens that with a small aperture and focal length.

Angular Magnification (M): M = $\frac{F_o}{F_e}$​​

Where $F_o$​ is the focal length of the objective lens and $F_eF$e​ is the focal length of the eyepiece lens.

Ray Optics and Optical Instruments Class 12 Notes Physics - Basic Subjective Questions

Section–A (One Mark Questions)

1. What do you mean by the twinkling effect of star light?

Ans. The twinkling effect of starlight is due to refraction of light from the star through the earth’s atmosphere. The light of the star has to travel through the fluctuating masses of earth’s atmosphere with varying temperature gradients so, the apparent position of the star fluctuates, and this gives rise to the twinkling effect of the star.

2. If an object moves towards a plane mirror with a speed v at an angle θ to the perpendicular to the plane of the mirror, find the relative velocity between the object and the image.

Ans.

Relative velocity of image w.r.t object in y-axis is zero. 

In x-axis,

$V_{R}=V\;cos\;\theta-(-v\;cos\;\theta )$

$V_{R}=2v\;cos\;\theta$

3. A man is 6 ft tall. In order to see his entire image, find the minimum length of a plane mirror that he requires.

Ans.

The length of the mirror does not depend on how far you are from the mirror. In order to see full image of a person, the minimum size of the mirror should be one half the person's height. This is so because, in reflection, the angle of incidence is equal to angle of reflection $1=\dfrac{h}{2}=\dfrac{6}{2}=3feet$

4. A ray of light passes through four transparent media with refractive indices $\mu _{1},\mu _{2},\mu _{3}\;and\;\mu_{4}$ as shown in the figure. The surfaces of all media are parallel. If the emergent ray CD is parallel to the incident ray AB, then which medium's refractive indices should be the same?

Ans. For successive refraction through different media $\mu\;sin\;\theta=$ constant. 

Here as θ is same in the two extreme media, $\mu _{1}=\mu_{4}$

5. A glass lens of refractive index 1.5 is placed in a trough of liquid. What must be the refractive index of the liquid in order to mark the lens disappear? 

Ans. In order to make the lens disappear the refractive index of liquid must be equal to 1.5 i.e. equal to that of glass lens.

Section–B (Two Marks Questions)

6. A glass slab is immersed in water. Find the critical angle at glass water interfaces, given aμg = 1.5 and aμw = 1.33.(Given $sin^{-1}(0\cdot 885)=62^{\circ}$ )

Ans. $_{w}\mu _{g}=\dfrac{1\cdot 5}{1\cdot 33}=1\cdot 13$

Using, $\dfrac{1}{sin\;C}=\mu$

$\dfrac{1}{sin\;C}=\dfrac{1}{1\cdot 13}$

$\Rightarrow C=62^{\circ}$

7. The refractive index of the material of the prism is $\sqrt{3}$ then find the angle of minimum deviation of the prism.

Ans. According to equation for refractive index of a prism,

$\mu =\dfrac{sin\left ( \dfrac{A+\delta }{2} \right )}{sin\left ( \dfrac{A}{2} \right )}\Rightarrow \sqrt{3}\dfrac{sin\left ( \dfrac{60^{\circ}+\delta }{2} \right )}{sin(30^{\circ})}$

$60^{\circ}+\delta=120^{\circ}$

$60^{\circ}=\delta$

8. A fish is a little away below the surface of a lake. If the critical angle is 49°, then the fish could see things above the water surface within an angular range of $\theta ^{\circ}$ . Find $\theta ^{\circ}$ .

Ans. 

From the figure, it is clear that, angular range is twice of the critical angle.

$\theta =2\times 49^{\circ}=98^{\circ}$

9. An object is placed at the midpoint of focus and the pole of a concave mirror. If the focal length of the mirror be f, then find the distance of the image from the pole of the mirror. 

Ans. Let, the focal length of the mirror be f, and the object distance is equal to focal length, $u=-\dfrac{f}{2}$

From mirror formula;

$-\dfrac{1}{f}=\dfrac{1}{V}+\dfrac{2}{-f}$

$\dfrac{1}{V}=\dfrac{1}{f}$

V=f

10. State the conditions for the phenomenon of total internal reflection to occur. 

Ans. Two essential conditions for total internal reflection are :

• Light should travel from an optically denser medium to an optically rarer medium.

• The angle of incidence in the denser medium must be greater than the critical angle for the two media.