Concave Lens Important Questions

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Very Short Answers Questions [1 Mark Questions]

Ques. Out of the following which one is a diverging or negative lens?

1. Plano-convex
2. Positive meniscus
3. Biconvex
4. Biconcave

Ans. The correct answer is d. Biconcave

Explanation: The diverging or negative lens is biconcave. Converging or positive lenses are biconvex, plano-convex, and positive meniscus.

Ques. The spherical surface of the lens results in

1. having a narrow range of focal length
2. having a narrow range of curvature
3. having a wide range of focal length
4. having a wide range of curvature

Ans. The correct answer is c. having a wide range of focal length

Explanation: Most lenses are composed of spherical surfaces because they are simple to create and have a wide range of curvature. As a result, the spherical surface of the lens results in a large range of curvature but not a great range of focal length.

Ques. From which part of the lens, the ray of light passes without deviation?

1. Centre of curvature
2. Optical center
3. Focus
4. Pole

Ans. The correct answer is b. Optical center

Explanation: The optical center is the portion of the lens through which the light beam travels without deviation.

Ques. A concave lens is commonly used in

1. Camera
2. Microscopes
3. Telescopes
4. Flashlights

Ans. The correct answer is c. Telescopes

Explanation: Concave lenses are commonly used in telescopes as eyepieces to magnify the image formed by the objective lens.

Ques. The power of the lens is the reciprocal of its

1. Principal axis
2. Focal length
3. Aperture
4. Optical center

Ans. The correct answer is b. Focal length

Explanation: The reciprocal of the focal length of the lens gives the power of the lens.

Ques. The point on the principal axis at the center of the lens is known as

1. Optical center
2. Pole
3. Focus
4. Power

Ans. The correct answer is a. Optical center

Explanation: The optical center is the point on the principal axis that is situated in the center of the lens.

Ques. Concave lens ______.

1. Converges the straight beam of light
2. Diverges the straight beam of light

Ans. The correct answer is b. Diverges the straight beam of light

Explanation: A diverging lens is another name for a concave lens. A straight beam of light is diverted by this lens.

Ques. The lens formula is given by

1. 1/f = 1/u - 1/v
2. 1/f = 1/v + 1/u
3. 1/f = 2/v - 1/u
4. 1/f = 1/v - 1/u

Ans. The correct answer is d. 1/f = 1/v - 1/u

Explanation: The lens formula is given by 

1/f = 1/v - 1/u

Ques. Which lens is thicker at the center and thinner at the edges?

1. Concave lens
2. Plane lens
3. Convex lens
4. Mirror

Ans. The correct answer is a. Concave lens

Explanation: The center of a concave lens is thicker than the edges.

Ques. Which type of lens is used in the Magnifying glass?

1. Convex lens
2. Concave lens

Ans. The correct answer is a. Convex lens

Explanation: Convex lenses are used in magnifying glasses because they make an object appear larger than it is.

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Short Answers Questions [2 Marks Questions]

Ques. Define a concave lens.

Ans. A concave lens has at least one surface that curves inwards. It is a divergent lens, which means it spreads out the light rays that have been refracted through it. Concave lenses produce upright images, diminished and virtual.

Ques. What are the different types of concave lenses?

Ans. The different types of concave lenses are

• Biconcave lens
• Plano-concave lens
• Convexo-concave lens

Ques. What is a lens?

Ans. A lens is a transmissive optical device that uses refraction to focus or disperse a light beam. A simple lens is made up of a single piece of transparent material, whereas a compound lens is made up of multiple simple lenses arranged along a common axis.

Ques. What happens when we look through a lens which is a concave lens?

Ans. A concave lens causes light rays to diverge in parallel and spread out such that they appear to originate from a point behind the lens which is the focal point. The focal length of a lens is defined as the distance between the center of the lens and a focus point.

Ques. Which type of image is formed by using a concave lens?

Ans. A concave lens, also known as a diverging lens, always forms a virtual, erect, and diminished image. This means that the image appears to be behind the lens, is upright, and is smaller than the object.

Ques. What is a mirror?

Ans. A mirror is an optical device that reflects light that strikes it. It is used to see object images. There are several types of mirrors, each with its unique function. The mirror is essentially a piece of transparent glass having two sides, one of which is coated with a reflecting substance and the other for reflection.

Ques. What is the focal length of a lens?

Ans. The focal length is the distance between the convex lens or concave mirror and the focal point of a lens or mirror. It is the point at which two parallel light beams meet or converge.

Ques. What is a convex lens?

Ans. The convex lens is a lens that converges light rays that are parallel to its principal axis (i.e. converges incoming rays towards the principal axis) and is comparatively thick in the center and thin at the edges. The edges are curved outward rather than inward.

Ques. What is the lens formula?

Ans. The Lens formula expresses the relationship between the distance of the image (v), the distance of the object (u), and the focal length (f) of the lens. The Lens formula applies to both convex and concave lenses. These lenses have negligible thickness.

The lens formula is given by 

1/f = 1/v - 1/u

Where

• f is the focal length of the lens
• u is the distance of the object
• v is the distance of the image

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Long Answers Questions [3 Marks Questions]

Ques. An object placed at 50 cm from a lens forms a real image at 80 cm on the other side of the lens. Find its focal length.

Ans. Given

• Distance of the object from the lens, u = - 50 cm
• Distance of the image from the lens, v = 80 cm

From the lens formula, we have

1/f = - 1/u + 1/v

On substituting the values, we get

1/f = - (1/-50) + (1/80)

⇒ f = + 30.8 cm

Therefore the focal length of the lens is 30.8 cm

Ques. A concave lens focuses an image 20 cm from the lens of an object at a distance of 30 cm from the lens. Find the focal length of the lens.

Ans. Given

• The distance of the object, u = -30 cm
• The distance of the image, v = -20 cm

Using the lens formula

1/f = 1/v - 1/u

On substituting the values, we get

1/f = (-1/20) - (-1/30)

⇒ 1/f = (-1/20) + (1/30)

⇒ f = -60 cm

Hence the focal length of the lens is -60 cm.

Ques. An object is placed 50 cm from a concave lens of focal length 20 cm, find the position and nature of the image.

Ans. Given

• Object distance, u = -50 cm
• Focal length of the concave lens, f = -20 cm

Using the lens formula

1/f = 1/v - 1/u

⇒ 1/v = 1/f + 1/u

On substituting the values, we get

1/v = (-1/20) + (-1/50)

⇒ v = -14.3 cm

The negative sign shows that the image is virtual and erect.

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Very Long Answers Questions [5 Marks Questions]

Ques. The radius of curvature of each face of a biconcave lens, made of glass with a refractive index of 1.55 is the same. Calculate the radius of curvature if the focal length of the lens in air is 20 cm.

Ans. Given 

• The focal length of the biconcave lens, f = -20 cm
• Refractive index of the glass, µ = 1.55

Using the lens maker formula

\(\frac{1}{f} = (\mu - 1)[ \frac{1}{R_1} - \frac{1}{R_2}]\)

Where

• R₁ is the radius of the curvature of the first refracting surface
• R₂ is the radius of the curvature of the second refracting surface

For biconcave lens, R₁ = -R and R₂ = +R

On substituting the values, we get

\(\frac{1}{-20} = (1.55-1) [-\frac{1}{R} - \frac{1}{R}]\)

⇒ \(\frac{1}{-20} =0.55 [-\frac{2}{R}]\)

⇒ R = 22 cm

Ques. Show that the minimum distance between a real object and its real image formed by a lens is equal to 4 times the focal length of the lens.

Ans. For a lens,

• The distance of a real object is taken as negative i.e. -u 
• The distance of a real image is taken as positive i.e. +v

L is the distance between the position of the object and its real image from the center of the lens, then

L = |u| + |v| …(i)

We can also above equation as

L= \((\sqrt{u} - \sqrt{v})^2+2 \sqrt{uv}\)

Now L will be the minimum if \((\sqrt{u} - \sqrt{v})^2\)=0

⇒ u = v …(ii)

From the lens formula, we have

- 1/u + 1/v = 1/f

Using the sign convention and equation (ii), the above equation becomes

- (1/-u) + (1/u) = 1/f

⇒ 2/u = 1/f

⇒ u = 2f

Substituting the above value in equation (i) and using equation (ii), we get

L = 2f + 2f = 4f

The above equation shows that the minimum distance between a real object and its real image formed by a lens is equal to 4 times the focal length of the lens.

Ques. What are the applications of concave lenses?

Ans. The following are the applications of concave lenses

• In telescopes: Concave lenses are used to magnify objects in telescopes and binoculars. Because a convex lens causes blurring and distortion, telescope and binocular makers put concave lenses before or in the eyepiece to improve focus.
• In eyeglasses: Concave lenses are commonly used to correct myopia, also known as nearsightedness. Myopia causes the eyeball to be too long, and images of distant objects fall short of the retina. As a result, concave lenses are used in spectacles to compensate for the lack by spreading out the light rays before they reach the eyeball. This improves the person's ability to see distant objects clearly.
• In Peepholes: Peepholes, sometimes known as door viewers, are security devices that provide a 360-degree view of objects beyond walls or doors. A concave lens is used to minimize the proportions of objects while providing a broader perspective of the object or area.
• Lasers: Concave lenses can be used in lasers to reflect the laser beam, causing parallel light rays to focus on a narrow beam. This enables lasers to be utilized for a wide range of purposes, including cutting, welding, and measurement.

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