Light: Reflection & Ref. Important Questions Class 10 Science with solutions

Light is a fascinating phenomenon that plays an important role in our everyday lives. Chapter 9, Light Reflection and Refraction, introduces you to the basic properties of light and how it interacts with surfaces and mediums. The chapter covers all important concepts, such as the laws of reflection and refraction, ray diagrams for mirrors and lenses, and the applications of optical phenomena in devices like periscopes, telescopes, and cameras.

Here’s an expanded blog to know this chapter—in and out—and prepare effectively for your exams, along with a list of  Important Questions of Light Class 10 to practice.

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Chapter 9 Light Reflection And Reflection Important Questions

1. Two convex lenses P and Q have focal length 0.50 m and 0.40 m respectively. Which of the following is TRUE about the combined power of the two lenses?

a. P is equal to 4.5 D.

b. P is less than 4.5 D.

c. P is more than 4.5 D.

d. P cannot be determined from the information given.

Answer: 

(a) P is equal to 4.5 D

Explanation:

P=P1​+P2​

P= 1/f

For lens P= 2D

For lens Q= 2.5D

P=P1​+P2​

=2+2.5

=4.5D

2. When an incident ray of light enters a medium from air, it bends towards the normal.

Which of the following is TRUE about the refractive index of the medium (n_m) as compared to the refractive index of air (n_a)?

a. n_m is equal to n_a. 

b. n_m is less than n_a

c. n_m is more than  n_a

d. (The refractive indices cannot be compared based on the given information.)

Answer:

(c) n_m is more than  n_a

Explanation:

When a ray of light enters a medium from air and bends towards the normal, it indicates that the speed of light is reduced in the medium. This happens because the refractive index of the medium (n_m​) is greater than the refractive index of air (n_a).

n_a​sini=n_m​sinr

 3. The image below shows a student demonstrating that sunrays concentrated to a point using a spherical mirror can burn a paper as a science project.

Answer the following questions.

3.1 What is the term used for the distance between the mirror and the paper?

a. radius of curvature

b. principal focus

c. principal axis

d. focal length

Answer:

‍(d) focal length

Explanation:

‍The paper is placed at the point where the sunrays converge after reflection from the mirror. This point is called the principal focus (F) of the mirror, and the distance between the mirror and this point is the focal length.

3.2 What kind of image would be formed on the paper?

a. real and diminished

b. real and enlarged

c. virtual and diminished

d. virtual and enlarged

Answer:

‍(a) real and diminished

Explanation:

The image formed on the paper is real because the sunrays physically converge to form the image. Real images can be projected onto a surface like paper.

The image of the Sun formed by the spherical mirror is diminished because the Sun is at a very large distance (essentially at infinity), and the image at the focal point is a small bright dot representing the concentrated rays.

4. If the student wishes to point the mirror to another object so as to obtain a virtual enlarged image, where should be the position of the object with respect to the mirror?

a. at principal focus

b. at centre of curvature

c. between pole and principal focus

d. between centre of curvature and principal focus

Answer:

(c) between pole and principal focus

Explanation:

When the object is between the pole and the principal focus, the reflected rays appear to diverge. If extended backward, they seem to originate from a point behind the mirror.

This creates a virtual, upright, and enlarged image, which can only be observed in this object position for a concave mirror.

5. Which of the following is NOT a common use for the type of spherical mirror used by the student for the experiment above?

a. car headlights

b. solar cooker

c. rear-view mirror

d. shaving mirrors

Answer:

‍(c) rear-view mirror

Explanation:

The experiment mentioned likely involves a convex mirror or a concave mirror, depending on the context. Let’s analyze each option based on the typical uses of these spherical mirrors:

• Car headlights: Use concave mirrors to focus light into a beam, making them efficient for projecting light over long distances.
• Solar cooker: Uses concave mirrors to concentrate sunlight at a focal point to generate heat.
• Rear-view mirror: Uses convex mirrors to provide a wider field of view for the driver.
• Shaving mirrors: Use concave mirrors to produce a magnified image when the face is close to the mirror.

The mirror used in the experiment cannot be used as a rear-view mirror, as it would typically require a convex mirror, which is not used for focusing experiments like solar cookers or headlights.

Thus, "rear-view mirror" is NOT a common use for the type of spherical mirror likely used in this experiment.

6. The eyeball of a person has become slightly larger. Which kind of lens should the person wear to correct the defect in the vision caused by this change in the size of the eyeball?

Answer:

If the eyeball of a person has become slightly larger, the increased size causes the retina to move farther from the eye's lens. As a result, the light rays converge in front of the retina rather than directly on it. This condition is known as myopia or nearsightedness.

To correct myopia, the person should wear a lens that diverges the light rays slightly before they enter the eye, ensuring they focus properly on the retina.

Correct Lens:

• The person should wear a concave lens (a diverging lens) with negative power.

This lens spreads the incoming light rays so that the eye's lens focuses them farther back, directly onto the retina, restoring clear vision.

7. In a human eye, the distance between the lens and the retina is 17 mm. The light entering the eye gets refracted at the cornea and then at the lens. Ciliary muscles in the eye can control the focal length of the lens by changing its shape.

(a) Diana is looking at the Moon. What is the focal length of the combination of cornea and the lens in Diana's eyes at this time?

(b) Diana is reading a book kept at a distance of 20 cm from her eyes. What is the focal length of the combination of the cornea and the lens in Diana's eyes at this time?

(c) When Diana brings the book closer to her eyes, the letters appear blurry to her and she cannot read the book. Explain why the letters appear blurry to her.

Answer:

(a)

When Diana is looking at the Moon, the Moon is essentially at infinity. For an object at infinity, the light rays entering the eye are parallel, and they converge at the retina.

The focal length (f) of the eye's optical system must match the distance between the lens and the retina to form a sharp image:

f=17mm

=0.017m

(b)

When Diana is reading a book at a distance of u=20 cm=0.2 m, the image is formed at the retina, so the image distance (v) is 17 mm (0.017 m).

1/f= 1/v-1/u

1/f= 1/0.017 - 1/-0.2

1/f= 58.82+5

=63.82

f= 1/63.82

≈0.0157m=15.7mm

(c)

When Diana brings the book closer, the distance between the book and her eyes (uuu) becomes smaller. For the eye to focus on such a close object, the focal length of the eye's lens system must decrease further. However, the eye's ciliary muscles have a limit to how much they can adjust the lens shape. If the required adjustment exceeds this limit:

• The image no longer forms on the retina.
• The letters appear blurry because the image forms either in front of or behind the retina.

This inability to adjust sufficiently is why the letters become blurry.

8. Smriti is looking at herself in a convex mirror in a science museum, standing 2 m away from the mirror. Her image appears to be around half her actual height. Estimate the focal length of the mirror.

Answer:

M= Height of image/Height of object

M= v/u

v= ½ . (-2)

v= −1m

The negative sign indicates that the image is virtual and behind the mirror.

1/f= 1/v - 1/u

1/f= -1 + ½ 

1/f= -½

f= -2m

9. A person needs a lens of power -5.0 D for correction of his vision.

(a) What is the possible vision defect of the person? 

(b) What is the focal length of the corrective lens?

Answer:

a. The power of the corrective lens is negative (−5.0 D), indicating it is a diverging lens. Diverging lenses are used to correct myopia (nearsightedness).

In myopia, the person can see nearby objects clearly but struggles to see distant objects. This occurs because the eye's lens focuses the image in front of the retina instead of on it. The diverging lens shifts the focal point back onto the retina, correcting the defect.

b. The relationship between the power PPP (in diopters) and the focal length fff (in meters) is:

P= 1/f

Substitute P=−5.0 D:

f= 1/(-5.0)

= −0.2m(or -20 cm)

10. The images formed by an ordinary convex lens suffer from a defect, called chromatic defect, which leads to false coloured edges in the images. This happens because light rays of different colours bend differently as they enter and leave the lens.

If a parallel white light beam passes through a convex lens, the light of which colour (among violet to red in the spectrum) will converge at a point closest to the lens? Justify your answer.

Answer:

When a parallel beam of white light passes through a convex lens, the phenomenon of chromatic aberration causes light of different colors (wavelengths) to refract by different amounts. This occurs because the refractive index of the lens material varies with the wavelength of light—a phenomenon known as dispersion.

Shorter wavelengths (violet): Light with shorter wavelengths, such as violet, experiences a higher refractive index. This means violet light bends more sharply and converges closer to the lens.

Longer wavelengths (red): Light with longer wavelengths, such as red, has a lower refractive index. This means red light bends less sharply and converges farther from the lens.

The light of violet color will converge at a point closest to the lens because it has the shortest wavelength in the visible spectrum and is refracted the most by the lens. This behavior is consistent with the dispersive nature of the lens material, leading to chromatic aberration.

Important Concepts in Reflection and Refraction Ch 9

As an essential phenomenon, light exhibits unique behaviors like reflection and refraction when interacting with surfaces and media. Understanding these concepts is important for mastering the chapter Light: Reflection and Refraction.

Reflection of Light - Reflection occurs when light rays strike a surface and bounce back into the same medium. This phenomenon sticks to the fundamental laws of reflection:

Laws of Reflection:

1. The angle of incidence equals the angle of reflection.
2. The incident ray, reflected ray, and normal (perpendicular to the surface at the point of incidence) all lie in the same plane.

Types of Reflection:

Regular Reflection:

• Occurs on smooth, polished surfaces like mirrors.
• Produces clear, well-defined images as the reflected rays are parallel.
• Example: Reflection from a plane mirror.

1. Diffuse Reflection:

• Happens on rough or uneven surfaces where the reflected rays scatter in multiple directions.
• Although the laws of reflection still hold at each point on the surface, the scattered nature of light prevents image formation.
• Example: Reflection from a wall or paper.

Refraction of Light - Refraction is the bending of light when it passes from one medium to another with different densities. Key topics include:

Laws of Refraction:

• The incident ray, refracted ray, and normal lie in the same plane.
• The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant (Snell’s Law).

Refractive Index:

• A measure of how much a medium bends light compared to a vacuum.
• Formula: n=cvn = \frac{c}{v}n=vc​, where ccc is the speed of light in vacuum and vvv is the speed in the medium.

Critical Angle and Total Internal Reflection (TIR):

• TIR occurs when the angle of incidence exceeds the critical angle, causing light to reflect completely within the denser medium.
• Applications include optical fibres and diamonds.

Ray Diagrams and Optical Devices / Ray Diagrams for Spherical Mirrors:

• Concave Mirrors: Focus on real and virtual image formation based on the object’s position (beyond C, at C, between C and F, etc.). Applications: Headlights, shaving mirrors, telescopes.

• Convex Mirrors: Always produce virtual, upright, and diminished images. Applications: Rearview mirrors in vehicles.

Ray Diagrams for Lenses:

• Convex Lenses: Analyse real and virtual image formation for different object positions. Applications: Magnifying glasses, microscopes, and cameras.

• Concave Lenses: Always form virtual, upright, and diminished images. Applications: Spectacles for myopia.

Applications of Reflection and Refraction

The principles of reflection and refraction form the foundation of numerous technologies and are integral to various scientific advancements. Here's a detailed look at their real-world applications:

Optical Instruments

Reflection and refraction are at the heart of many optical instruments used in science and day-to-day life:

• Periscopes and Telescopes: Reflection helps redirect light using mirrors, enabling vision over obstacles (periscopes) or capturing distant celestial objects (telescopes).
• Microscopes and Cameras: Refraction allows lenses to magnify objects in microscopes for detailed analysis or focus light onto a camera sensor to create clear images.
• Magnifying Glasses: Convex lenses refract light to magnify objects for close examination.

Everyday Devices

• Mirrors in Vehicles: Convex mirrors are used for rear-view mirrors, providing a wider field of view to enhance road safety.
• Spectacles: Lenses in glasses correct refractive errors:

• Concave Lenses: For myopia (nearsightedness), they diverge light to focus on the retina.
• Convex Lenses: For hypermetropia (farsightedness), they converge light for better focus.

• Smartphone Cameras: Lenses and mirrors inside cameras use refraction and reflection to improve image quality.

Communication

Optical Fibers: Total Internal Reflection (TIR) enables optical fibers to transmit data as light signals with minimal loss. It is widely used in internet cables, medical endoscopes, and telecommunication networks for efficient and high-speed data transfer.

Astronomy

Telescopes: Reflecting and refracting telescopes rely on mirrors and lenses to magnify and observe distant celestial bodies. Large astronomical telescopes use parabolic mirrors for high-precision reflection, capturing faint light from stars and galaxies.

Advanced Technologies

Virtual Reality (VR) and Augmented Reality (AR): Devices use reflection and refraction principles for immersive visual experiences by projecting light at precise angles.

Laser Applications: Refraction focuses laser beams for cutting, surgery, or scientific experiments, while reflection ensures device beam direction.

How to Prepare for Light – Reflection and Refraction

Master the Laws and Definitions

• Memorize the laws of reflection and refraction along with their real-life examples.
• Understand critical terms like focal length, principal axis, refractive index, and critical angle.

Practice Ray Diagrams

• Practice drawing and labeling ray diagrams for mirrors and lenses.
• Ensure clarity in accurately marking the focal point, center of curvature, and image formation.
• Focus on learning the rules for constructing ray diagrams for concave and convex mirrors and lenses. 

Focus on Applications

• Study the practical applications of concepts like total internal reflection (e.g., optical fibers, diamond brilliance).
• Understand the uses of concave and convex mirrors in everyday life. 

Revise and Practice Regularly

• Solve NCERT exercises, exemplar problems, and additional sample papers to reinforce your understanding.
• Time yourself while solving numerical problems to improve speed and accuracy.

Clarify Doubts

• Seek help from teachers or peers for challenging topics like ray diagrams, refraction at curved surfaces, or TIR.
• Use online tutorials and animations to visualize complex phenomena.

Chapter 9 Light CBSE Class 10 bridges theoretical knowledge with practical applications, making it an important topic for exams and daily life. You can confidently tackle any question in this chapter by understanding the key concepts, mastering ray diagrams, solving Light Reflection And Refraction Class 10 Extra Questions and numerical problems, and revising applications.

Stay consistent in your preparation and keep practicing to shine bright, just like light!