CHAPTER # 12
GEOMETRICAL OPTICS

Multiple Choice Questions

1. Its Frequency.
2. - 15cm
3. at the center of curvature.
4. - 9.9cm
5. the speed of light.
6. inverted and virtual.
7. virtual, inverted, magnified.
8. real, inverted and diminished.
9. reflect only.
10. totally reflected.

 REVIEW QUESTIONS

12.1. What do you understand by Reflection of light? Draw a diagram to illustrate reflection at a plane surface.

Reflection of light occurs when light traveling in one medium falls on the surface of another medium and a portion of it returns to the same medium.

12.2. Describe the following terms used in Reflection.

Term	Definition
Normal	At the point of incidence, the perpendicular is referred to as normal.
Angle of Incidence	The angle with which a light ray strikes a reflecting surface.
Angle of Reflection	The angle is formed when a ray reflects off of a surface and intersects with the normal drawn there.

 

12.3. State Laws of Reflection? Describe how they can be verified graphically.

• The incident ray, the normal, and the reflected ray at the point of incidence all lie in the same plane.
• The angle of incidence is equal to the angle of reflection.

12.4. Define Refraction of Light? Describe the passage of light through a parallel-sided transparent material.

Refraction of light is the term used to describe how light bends as it travels through transparent materials.

 12.5. Define the following terms used in refraction.

Term	Definition
Angle of incidence	The angle of incidence is the angle that the incident ray forms with the normal.
Angle of refraction	The angle of refraction is the angle formed by the refracted ray and the normal line.

 12.6. What is meant by the Refractive index of a material? How would you determine the refractive index of a rectangular glass slab?

The ratio of the sin of the angle of incidence to the sin of the angle of refraction remains constant as a light ray travel from one medium to another. The refractive index is the name given to this constant ratio.

12.7. State the Laws of Refraction of Light? and show how they may be verified using rectangular glass slabs and pins.

·         The incident ray, the normal, and the reflected ray at the point of incidence all lie in the same plane.

·         The ratio of the sin of the angle of incidence to the sin of the angle of refraction is constant.

 12.8. What is meant by the term Total Internal reflection?

No refraction occurs when the angle of incidence exceeds the critical angle. The light is completely reflected into the denser medium. This is referred to as Total Internal Reflection.

 12.9. State the conditions for Total Internal reflection?

·         The light ray travels from a denser medium to a less dense medium.

·         In the denser medium, the angle of incidence is greater than the critical angle.

 12.10. What is Critical Angle? Derive a relationship between the critical angle and the refractive index of a substance.

The critical angle is the angle of incidence at which the angle of refraction becomes 90 degrees.

12.11. What are Optical Fibers? Describe how total internal reflection is used in light propagating through optical fibers.

“An optical fiber cable is a bundle of glass fibers with the thickness of a human hair”.

Light will repeatedly undergo total internal reflection at the walls if it enters from one end with an angle of incidence greater than the critical angle and exits at the other end with no loss of intensity.

 12.12. Define the following terms applied to a lens.

Term	Definition
Principal axis	The line passing through the two centers of curvatures of the lens is called the principal axis.
Optical center	A point on the principal axis at the centre of the lens is called the optical center.
Focal length	The focal length is the distance between the optical center and the principal focus.

  12.13. What is meant by the principal focus of the following?

Principal focus of convex lens	Principal focus of concave lens
After refraction, light rays traveling parallel to the principal axis of a convex lens meet at a point on the principal axis known as the principal focus or focal point. As a result, a convex lens is also known as a converging lens.	The parallel rays appear to emit from a location behind the lens known as the principal focus. As a result, a concave lens is also known as a diverging lens.

  12.14. Describe how light is Refracted through the convex lens.

·     After refraction by the lens, the ray parallel to the principal axis passes through the focal point.

·    The ray that passes through the optical center passes straight through the lens with no deviation.

·  After refraction by the lens, the ray passing through the focal point becomes parallel to the principal axis.

 12.15. With the help of a ray diagram, how can you show the use of a thin converging lens as a magnifying glass?

A magnifying glass is a lens that creates a virtual image larger than the object that appears behind the lens.

12.16. A coin is placed at a focal point of a converging lens. Is an image formed? What is its nature?

A coin is placed at the converging lens's focal point. The refracted rays are parallel and never cross, so no image is created.

12.17. What are the Differences between Real and Virtual Images?

Real Image	Virtual Image
The real image usually appears inverted.	The virtual image usually appears erect.
Real images can be obtained on a screen.	A virtual image cannot be obtained on a screen.
Light rays meet at a focal point in front of the mirror.	Light rays meet at a focal point behind the mirror.
In the mirror, Real images lie in front of the reflecting surface.	In the mirror, a virtual image lies behind the mirror.
On the lens, Real images lie on the other side of the object.	On the lens, Virtual images lie on the same side of the object.

 12.18. How does a converging lens form a virtual image of a Real object? How can a diverging lens form a real image of a Real object?

·     A virtual image of a real object can be created by a convergent lens. A virtual image may be obtained by positioning the object between the lens and the main focus.

·    Since a diverging or concave lens' light rays diverge as you extend them and never end up intersecting, they cannot create a real image of an actual object.

 12.19. Define the power of a lens and its Units.

The reciprocal of a lens's focal length in meters is defined as its power. Its unit is dioptre, which is represented by the letter D and equals per meter.

12.20. Describe the passage of light through a glass prism and measure the angle of deviation?

Ray of light PQ strikes at E and refracted toward F and again further deviates from F as an emergent ray RS. Extend both PQ and RS rays backward. Both rays will meet at point G. At this point, they will make an angle of deviation D.

12.21. Define the terms Resolving Power and Magnifying Power.

Resolving Power	Magnifying Power
The resolving power of an instrument is its ability to show the minor details of the object under examination.	The ratio of the angles occupied by the image as seen through the optical device to the angles occupied by the object as seen with the unaided eye.

 12.22. Draw the ray diagram of……….

Simple Microscope (figure 12.30 b)

Compound Microscope (figure 12.32)

Refracting Telescope (figure 12.34)

12.23. Mention the magnifying power of the following.

12.24. Draw ray diagrams to show the formation of images in the normal human eye.

(Figure 12.36 a and b)

12.25. What is meant by the terms Nearsightedness and Farsightedness? How can these defects be corrected?

Nearsightedness (myopia)	Farsightedness (hypermetropia)
Some people require spectacles to see distant objects clearly. This vision problem is known as nearsightedness or short sight.	Farsightedness is the inability of the eye to form distinct images of nearby objects on its retina.
Correction of Nearsightedness	Correction of Farsightedness
Nearsightedness can be corrected with diverging lenses in glasses or contact lenses.	A suitable converging lens can be used to correct farsightedness.

 CONCEPTUAL QUESTIONS

 12.1.   A man raises his left hand in a plane mirror the image facing him is raising his right hand. Explain why?

In a plane mirror, a man raises his left hand, while the image opposite him raises his right hand. Because light rays are reflected in the mirror, we see an inverted image.

 12.2.   In your own words, explain why light waves are refracted at a boundary between two materials.

Because of the change in the speed of light as it travels from one medium to another, light waves are reflected at the boundary between two materials. The change in light speed is caused by a change in the refractive indices of two materials.

 12.3.   Explain why a fish underwater appears to be at a different depth below the surface than it actually is. Does it appear deeper or shallower?

A fish underwater appears shallower due to apparent depth as a virtual image is formed above the object (fish) due to light refraction.

 12.4.   Why or why not are concave mirrors suitable for makeup?

Concave mirrors are ideal for makeup because they act as magnifiers, forming large erect images of objects. It occurs when a person stands between the principal focus and the pole of the mirror.

 12.5.   Why is the driver’s side mirror in many cars convex rather than plane or concave?

The driver's side mirror is convex because it provides an upright, erect image; however, because it is curved outwards, it provides a wider field of view.

 12.6.   When an optician's testing room is small, he uses a mirror to help him test the eyesight of his patients. Explain why?

If the optician's eye testing room is small, he will use a plane mirror to increase the distance between the alphabet and the patients.

 12.7.   How does the thickness of the lens affect its focal length?

The power of the lens increased as the thickness of the lens increased. Because power and focal length have an inverse relationship, the focal length decreases.

 12.8.   Under what conditions will a converging lens form a virtual image?

When an object is placed between the optical center and the focal point of a convergent lens, a virtual image is formed. The formed image will be upright and larger than the object.

 12.9.   Under what conditions will a converging lens form a real image that is the same size as the object?

When an object is placed at 2F from the converging lens, a real and inverted image of the same size as the object is formed.

 12.10. Why do we use a refracting telescope with a large objective lens of a large focal length?

In a refracting telescope, we use a larger focal length objective lens to gather more light from a weak distant source such as stars, moon, etc. This not only makes them visible but also increases the resolving power of the telescope.

THANKS!!!

STAY UPDATED!!!