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Science Grade XII Physics

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Chapter 9: Ray Optics and Optical Instruments

Grade 12 Science | Chapter 9

Ray Optics and Optical Instruments

Light travels in rays that reflect and bend. This chapter develops reflection and mirrors, refraction and Snell’s law, lenses and image formation, and dispersion by a prism.

Core Concepts

Key Principles

Worked Examples

Practice Sets

Contents

Introduction: Rays of Light

Reflection and Mirrors

Refraction and Snell’s Law

Lenses and Images

Dispersion by a Prism

Optical Instruments

Key Reasoning (Principles)

Worked Examples (10)

Practice Sets A to D

10.

Summary and Exam Quick-Check

1. Introduction: Rays of Light

In many situations light travels in straight lines that we can draw as rays. Ray optics uses these straight rays to explain how light reflects from mirrors and refracts, or bends, when it passes between materials. From these two simple behaviours we can understand mirrors, lenses, the eye, and instruments like the microscope and telescope. This chapter follows the rules that rays obey.

Core idea

Light reflects so that the angle of incidence equals the angle of reflection, and refracts, or bends, when its speed changes between materials, following Snell’s law.

2. Reflection and Mirrors

When light strikes a surface it reflects, and the angle of incidence equals the angle of reflection, both measured from the normal. A flat mirror gives an upright image the same size as the object, behind the mirror. A curved mirror can converge or spread the rays: a concave mirror can form a real, magnified image, which is why it is used in shaving mirrors and telescopes.

3. Refraction and Snell’s Law

When light passes from one material to another its speed changes, so it bends, an effect called refraction. Going into a denser medium, such as from air into glass, the ray bends toward the normal. The bending follows Snell’s law, n1 times sin i equals n2 times sin r, where n is the refractive index of each medium. Refraction is why a straw in water looks bent.

Diagram 1 – Refraction of Light

$A light ray refracting and bending toward the normal as it enters glass$

Fig 1. Entering a denser medium, the ray bends toward the normal, following Snell’s law.

4. Lenses and Images

A lens uses refraction to bring rays together or spread them apart. A convex lens converges parallel rays to a focus, and can form a real, inverted image of a distant object. By tracing two standard rays, one parallel to the axis then through the focus, and one straight through the centre, we can find where the image forms and how large it is. A concave lens spreads rays and gives an upright, smaller image.

Diagram 2 – Convex Lens Image

A convex lens forming a real inverted image of an object

Fig 2. A convex lens brings rays from the object to a focus, forming a real, inverted image.

5. Dispersion by a Prism

White light is a mixture of colours, and a prism can separate them. Because each colour bends by a slightly different amount, the prism spreads white light into a band of colours, from red to violet, called a spectrum. This splitting is called dispersion, and it is the same effect that makes a rainbow when sunlight passes through raindrops.

Diagram 3 – Dispersion by a Prism

A prism dispersing white light into a spectrum of colours

Fig 3. A prism bends each colour by a different amount, spreading white light into a spectrum.

6. Optical Instruments

Lenses and mirrors are combined to build optical instruments. The eye focuses light with a lens onto the retina. A magnifying glass is a single convex lens that makes nearby things look larger. The microscope uses two lenses to magnify tiny objects, and the telescope uses lenses or mirrors to bring distant objects into view. Each instrument applies the same rules of refraction and reflection.

7. Key Reasoning (Principles)

Principle 1: Reflection keeps the angle

Light reflects so that the angle of incidence equals the angle of reflection, both measured from the normal to the surface.

Principle 2: Refraction bends light when speed changes

When light enters a denser medium it slows and bends toward the normal, following Snell’s law n1 sin i equals n2 sin r.

Principle 3: Rays can be traced to find images

By drawing a few standard rays through a lens or mirror, the position and size of the image can be found geometrically.

8. Worked Examples

Example 1

Q: State the law of reflection.

▶ Show Solution

The angle of incidence equals the angle of reflection.

Answer: Angle of incidence equals angle of reflection.

Example 2

Q: What kind of image does a flat mirror give?

▶ Show Solution

An upright image, the same size, behind the mirror.

Answer: Upright, same size, behind.

Example 3

Q: What is refraction?

▶ Show Solution

The bending of light when its speed changes between materials.

Answer: Bending when speed changes.

Example 4

Q: Which way does light bend entering a denser medium?

▶ Show Solution

Toward the normal.

Answer: Toward the normal.

Example 5

Q: Write Snell’s law.

▶ Show Solution

n1 times sin i equals n2 times sin r.

Answer: n1 sin i = n2 sin r.

Example 6

Q: What kind of image does a convex lens form of a distant object?

▶ Show Solution

A real, inverted image.

Answer: Real and inverted.

Example 7

Q: What is dispersion?

▶ Show Solution

The splitting of white light into its colours.

Answer: Splitting into colours.

Example 8

Q: Why does a prism disperse light?

▶ Show Solution

Because each colour bends by a slightly different amount.

Answer: Colours bend by different amounts.

Example 9

Q: What does a magnifying glass use?

▶ Show Solution

A single convex lens.

Answer: A convex lens.

Example 10

Q: Name two instruments that use lenses or mirrors.

▶ Show Solution

The microscope and the telescope (also the eye).

Answer: Microscope and telescope.

9. Practice Sets A to D

Set A – Multiple Choice (Basic)

1. In reflection, the angle of incidence equals the angle of: (a) refraction (b) reflection (c) normal (d) prism

2. Entering glass from air, light bends: (a) away from normal (b) toward normal (c) not at all (d) backward

3. Snell’s law is: (a) n1 sin i = n2 sin r (b) n1 i = n2 r (c) sin i = sin r (d) n1 = n2

4. A convex lens forms, of a distant object, a: (a) virtual upright image (b) real inverted image (c) no image (d) larger upright image

5. Splitting white light into colours is: (a) reflection (b) dispersion (c) diffraction only (d) absorption

▶ Reveal Answers

1. (b) reflection.

2. (b) toward normal.

3. (a) n1 sin i = n2 sin r.

4. (b) real inverted image.

5. (b) dispersion.

Set B – Short Answer (Understanding)

1. State the law of reflection.

2. Explain refraction and which way light bends into a denser medium.

3. Write Snell’s law and name the symbols.

4. Describe the image formed by a convex lens of a distant object.

5. Explain why a prism disperses white light.

▶ Reveal Answers

1. The angle of incidence equals the angle of reflection, measured from the normal.

2. Refraction is the bending of light when its speed changes; entering a denser medium it bends toward the normal.

3. n1 times sin i equals n2 times sin r; n is refractive index, i the angle of incidence, r the angle of refraction.

4. A real, inverted image on the far side of the lens.

5. Because each colour bends by a slightly different amount, so they spread into a spectrum.

Set C – Application and Reasoning

1. Why does a straw in water look bent?

2. Why can a concave mirror magnify your face up close?

3. Why does light bend toward the normal entering glass?

4. Why is a convex lens used as a magnifying glass?

5. Why does a rainbow form from sunlight and rain?

▶ Reveal Answers

1. Because light refracts as it leaves the water, bending so the straw appears displaced.

2. Because it converges the rays to form an enlarged image of a nearby object.

3. Because light slows in the denser glass, which makes it bend toward the normal.

4. Because it converges rays so that a nearby object appears larger.

5. Because raindrops refract and disperse the sunlight into its colours, like tiny prisms.

Set D – Higher Order (Challenge)

1. Explain how tracing two rays locates the image from a convex lens.

2. Explain why refraction occurs only when the speed of light changes.

3. Explain how dispersion shows that white light is a mixture.

4. Explain why the same rules build both a microscope and a telescope.

5. A ray enters glass and bends toward the normal. Explain what happens as it leaves back into air.

▶ Reveal Answers

1. One ray parallel to the axis passes through the focus, and one through the centre goes straight; where they meet is the image.

2. Because bending comes from the change in speed at the boundary, so if the speed did not change there would be no bending.

3. Because the prism separates white light into many colours, showing that those colours were present together all along.

4. Both combine lenses or mirrors that refract and reflect light by the same laws, arranged to magnify either tiny or distant objects.

5. Leaving the denser glass for air it speeds up and bends away from the normal, the reverse of entering.

Chapter Summary

Reflection

Angle of incidence equals angle of reflection.

Refraction

Light bends when its speed changes between materials.

Snell’s Law

n1 times sin i equals n2 times sin r.

Convex Lens

Converges rays; real inverted image of a distant object.

Dispersion

A prism splits white light into a spectrum.

Instruments

Eye, magnifier, microscope and telescope.

Quantity

Unit

Symbol

Reflection

i = reflection angle

–

Snell’s law

n1 sin i = n2 sin r

–

Convex image

real, inverted

–

8-Point Exam Quick-Check

Reflection: angle of incidence equals angle of reflection.

Refraction: light bends when its speed changes between materials.

Into a denser medium, light bends toward the normal.

Snell’s law: n1 times sin i equals n2 times sin r.

A convex lens gives a real, inverted image of a distant object.

A prism disperses white light into a spectrum.

Rays can be traced to find an image’s position and size.

Microscopes and telescopes apply the same optical rules.

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Class 12 Physics Chapter 9: Ray Optics and Optical Instruments, Complete Notes and Practice

This revision guide follows the current NCERT Class 12 Physics syllabus and develops ray optics, covering reflection and mirrors, refraction and Snell’s law with the refractive index, image formation by convex and concave lenses, dispersion of white light by a prism, and the optical instruments such as the eye, microscope and telescope, with three diagrams, ten worked examples and graded practice. Visit SchoolRevise.com to revise, practise and excel.