Class 12 Physics Chapter 9 Ray Optics and Optical Instrument

Chapter 9 of Class 12 Physics, titled “Ray Optics and Optical Instruments,” delves into the fundamental principles of light propagation, reflection, and refraction through various media. The chapter starts with the concept of light as a ray and builds upon the laws of reflection and refraction, explaining how light interacts with different surfaces and mediums.

Class 12 Physics Chapter 9 Ray Optics and Optical Instrument Book 📚 download in PDF format 👇👇

The chapter covers key topics such as the reflection of light by plane and spherical mirrors, the refraction of light through lenses, and the formation of images by mirrors and lenses. It also explains the concept of total internal reflection, a phenomenon that has practical applications in fiber optics and optical instruments.

Further, the chapter explores optical instruments like microscopes and telescopes, explaining their construction, working principles, and the role of lenses and mirrors in these devices. The chapter concludes with a discussion on the dispersion of light, optical fibers, and the defects of vision along with their correction using lenses.

Questions with Answers

1. What is the principle of reflection of light?

The principle of reflection states that when light is incident on a smooth surface, the angle of incidence is equal to the angle of reflection, and the incident ray, the reflected ray, and the normal lie in the same plane.

2. State the laws of refraction.

The laws of refraction state that (i) the incident ray, the refracted ray, and the normal to the interface of two media lie in the same plane, and (ii) the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant, known as Snell’s law.

3. What is Snell’s law?

Snell’s law relates the angles of incidence and refraction for a wave crossing the interface between two different isotropic media. It is mathematically expressed as ( n_1 \sin \theta_1 = n_2 \sin \theta_2 ), where ( n_1 ) and ( n_2 ) are the refractive indices of the two media.

4. Define the term ‘total internal reflection’.

Total internal reflection occurs when a light ray traveling from a denser to a rarer medium is incident at an angle greater than the critical angle, causing it to be completely reflected back into the denser medium.

5. What is the critical angle?

The critical angle is the angle of incidence in the denser medium at which the angle of refraction in the rarer medium becomes 90°, causing the refracted ray to travel along the boundary.

6. Explain the working of a simple microscope.

A simple microscope uses a convex lens to produce a magnified image of a small object. The object is placed within the focal length of the lens, resulting in a virtual, erect, and magnified image.

7. What is the magnification produced by a convex lens?

The magnification ( m ) produced by a convex lens is the ratio of the height of the image to the height of the object, given by ( m = \frac{v}{u} ), where ( v ) is the image distance and ( u ) is the object distance.

8. How does a concave mirror form an image?

A concave mirror forms an image based on the position of the object relative to its focal point. Depending on the position, the image can be real and inverted or virtual and erect.

9. What is the focal length of a mirror?

The focal length of a mirror is the distance between the mirror’s pole and its principal focus. For concave mirrors, the focal length is positive, and for convex mirrors, it is negative.

10. Describe the phenomenon of dispersion of light.

Dispersion of light occurs when white light passes through a prism, splitting into its constituent colors due to varying refractive indices for different wavelengths, forming a spectrum.

11. What is an optical fiber and its application?

An optical fiber is a thin, flexible strand of glass or plastic that transmits light signals with minimal loss, primarily used in telecommunications for high-speed data transmission.

12. How does a compound microscope work?

A compound microscope uses two convex lenses (the objective and eyepiece) to produce a highly magnified image of a small object. The objective lens creates a real, inverted image, which is further magnified by the eyepiece.

13. What is the resolving power of an optical instrument?

The resolving power of an optical instrument is its ability to distinguish between two closely spaced objects. It depends on the wavelength of light used and the aperture of the instrument.

14. What are spherical aberrations in lenses?

Spherical aberration is an optical defect where rays passing through the edges of a lens focus at different points than those passing through the center, causing a blurred image.

15. What is the difference between a real image and a virtual image?

A real image is formed when light rays converge at a point, and it can be projected onto a screen. A virtual image is formed when light rays appear to diverge from a point and cannot be projected.

16. How does a refracting telescope differ from a reflecting telescope?

A refracting telescope uses lenses to form an image, while a reflecting telescope uses a concave mirror to gather and focus light. Reflecting telescopes are generally free from chromatic aberration.

17. What is chromatic aberration in lenses?

Chromatic aberration is the failure of a lens to focus all colors to the same point, resulting in a blurred image with colored fringes. It occurs due to the variation in refractive index for different wavelengths of light.

18. Explain the concept of optical path length.

Optical path length is the product of the refractive index of a medium and the physical distance light travels in that medium. It represents the distance light would travel in a vacuum in the same time.

19. What is the power of a lens?

The power of a lens is the reciprocal of its focal length, measured in diopters (D). It indicates the lens’s ability to converge or diverge light.

20. What is the purpose of using a prism in optical instruments?

A prism is used in optical instruments to disperse light into its constituent colors, deviate light rays, or reflect light internally through total internal reflection.

21. What is astigmatism and how is it corrected?

Astigmatism is a common vision defect where the eye cannot focus light evenly on the retina due to an irregular curvature of the cornea or lens. It is corrected using cylindrical lenses that compensate for the uneven curvature.

22. How does the eye lens form an image?

The eye lens forms a real, inverted image on the retina by adjusting its focal length through accommodation, allowing clear vision of objects at varying distances.

23. What is the far point and near point of the human eye?

The far point is the maximum distance at which the eye can see clearly, typically infinity. The near point is the minimum distance at which the eye can focus, usually around 25 cm for a normal eye.

24. Explain the principle of a Galilean telescope.

A Galilean telescope uses a convex objective lens and a concave eyepiece lens to produce an upright image. It is simple and provides a narrow field of view, mainly used for terrestrial observations.

25. What is the phenomenon of diffraction of light?

Diffraction of light is the bending of light waves around obstacles or the spreading of light waves as they pass through a narrow aperture, leading to interference patterns.