Reflection and Refraction, Spherical Mirrors and Lenses
Reflection and Refraction, Spherical Mirrors and Lenses MCQ Questions
1.
What type of image is formed by a convex lens when the object is placed beyond the focal point?
Approach
The question asks about the nature of the image formed by a convex lens when the object is placed beyond its focal point. To answer this, we need to recall the ray tracing rules for convex lenses and the characteristics of images formed at different object positions.
Step-by-step
- Understanding Convex Lenses: A convex lens is a converging lens, meaning it converges parallel rays of light to a single point (its principal focus).
- Object Beyond Focal Point (F): When an object is placed beyond the focal point (but not necessarily at infinity) of a convex lens, the rays of light originating from the object pass through the lens and converge to form an image.
- Ray Tracing:
- A ray parallel to the principal axis passes through the principal focus (F) after refraction.
- A ray passing through the optical center (O) goes undeviated.
- A ray passing through the principal focus (F) becomes parallel to the principal axis after refraction.
- Image Formation: When these refracted rays intersect, they form an image. For an object placed beyond the focal point of a convex lens, the rays converge on the other side of the lens.
- Characteristics of the Image: The image formed is typically:
- Real: Because the light rays actually converge and intersect at a point to form the image, it can be projected onto a screen.
- Inverted: The image is upside down relative to the object.
- Diminished or Magnified: The size of the image depends on the exact position of the object (e.g., between F and 2F, or beyond 2F), but it is always real and inverted. For an object beyond F, but before 2F, the image is magnified. For an object beyond 2F, the image is diminished. The question only specifies "beyond the focal point," which covers both cases where the image is real and inverted.
Therefore, when an object is placed beyond the focal point of a convex lens, the image formed is real and inverted.
Correct Option
A) Real and inverted — This is the correct description of the image formed by a convex lens when the object is placed beyond its focal point. The image is formed by the actual intersection of refracted rays, making it real, and it is oriented upside down, making it inverted.
Incorrect Options
- B) Real and erect — A real image is always inverted for a single convex lens. An erect image formed by a convex lens is always virtual (when the object is placed between the optical center and the focal point).
- C) Virtual and inverted — Virtual images are always erect for a single lens. An inverted image is always real. These two characteristics (virtual and inverted) do not occur together for a single lens.
- D) Virtual and erect — A virtual and erect image is formed by a convex lens only when the object is placed between the optical center and the focal point. When the object is beyond the focal point, the image is real and inverted.
2.
What is the unit of measurement for the power of a lens?
Approach
The question asks for the unit of measurement for the power of a lens. We need to recall the definition of lens power and its standard unit in the International System of Units (SI).
Step-by-step
- The power of a lens is a measure of its ability to converge or diverge light rays. It is defined as the reciprocal of its focal length.
- Mathematically, the power of a lens (\(P\)) is given by the formula: \[ P = \frac{1}{f} \] where \(f\) is the focal length of the lens in meters.
- Since focal length is measured in meters (m), the unit of power will be reciprocal meters, or \(\text{m}^{-1}\).
- This unit, \(\text{m}^{-1}\), is specifically named the Dioptre (symbol D).
- Therefore, if a lens has a focal length of 1 meter, its power is 1 Dioptre.
Correct Option:
A) Dioptre is the correct unit for the power of a lens. It is defined as the reciprocal of the focal length in meters.
Incorrect Options
- B) Watt: This is the SI unit of power, but it refers to electrical or mechanical power (rate of doing work or transferring energy), not the optical power of a lens. One Watt is equal to one Joule per second (\(1 \text{ W} = 1 \text{ J/s}\)).
- C) Newton: This is the SI unit of force. It is defined as the force required to accelerate a mass of one kilogram at a rate of one meter per second squared (\(1 \text{ N} = 1 \text{ kg} \cdot \text{m/s}^2\)).
- D) Joule: This is the SI unit of energy or work. It is defined as the work done when a force of one Newton displaces an object by one meter (\(1 \text{ J} = 1 \text{ N} \cdot \text{m}\)).
3.
Which of the following is NOT a type of lens?
ANSWER :
D. Reflecting lens
Approach
The question asks to identify which option is NOT a type of lens. We need to understand the basic types of lenses and their functions in optics.
Step-by-step
- Understand what a lens is: A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. It is typically made of a transparent material like glass or plastic.
- Review common types of lenses:
- Convex lens: Also known as a converging lens, it is thicker in the middle and thinner at the edges. It converges parallel rays of light to a single focal point.
- Concave lens: Also known as a diverging lens, it is thinner in the middle and thicker at the edges. It diverges parallel rays of light.
- Diverging lens: This is another name for a concave lens, as it causes parallel light rays to spread out or diverge.
- Consider the term "Reflecting lens": The primary function of a lens is based on the principle of refraction (bending of light as it passes through a medium). Reflection, on the other hand, is the bouncing back of light from a surface, which is the principle behind mirrors. While there are optical systems that combine lenses and mirrors (e.g., catadioptric systems), a "reflecting lens" as a standalone type of lens is not a standard term. Optical devices that primarily use reflection are called mirrors.
- Compare with the given options: Options A, B, and C all describe actual types or functions of lenses (convex, concave, diverging). Option D, "Reflecting lens," describes a device that would primarily use reflection, which is characteristic of a mirror, not a lens.
Therefore, "Reflecting lens" is not a type of lens.
Correct Option:
D) Reflecting lens — Lenses work on the principle of refraction, bending light as it passes through them. Reflecting surfaces are characteristic of mirrors, not lenses. While some optical systems combine reflective and refractive elements (catadioptric systems), "reflecting lens" is not a recognized type of lens itself.
Incorrect Options
- A) Convex lens — This is a standard type of lens that converges light rays.
- B) Concave lens — This is a standard type of lens that diverges light rays.
- C) Diverging lens — This is another name for a concave lens, describing its function of spreading out light rays.
4.
What is the focal point of a concave mirror?
A.
The point where parallel rays of light converge
B.
The point where diverging rays of light converge
C.
The point where parallel rays of light diverge
ANSWER :
A. The point where parallel rays of light converge
Approach
The question asks for the definition of the focal point of a concave mirror. We need to recall the fundamental properties of how concave mirrors reflect light rays.
Step-by-step
- A concave mirror is a converging mirror, meaning it brings parallel rays of light together after reflection.
- When parallel rays of light (typically coming from a distant object) strike the surface of a concave mirror, they reflect off the curved surface.
- Due to the curvature of the mirror, all these reflected rays converge and meet at a single point on the principal axis.
- This specific point where the parallel rays of light converge after reflection from a concave mirror is defined as its focal point (or principal focus).
Correct Option: A) The point where parallel rays of light converge
This statement accurately defines the focal point of a concave mirror. Concave mirrors are known for their converging property, especially for parallel incident rays.
Incorrect Options
- B) The point where diverging rays of light converge — This is not the standard definition of a focal point for a concave mirror. While a concave mirror can converge diverging rays, the definition of the focal point specifically refers to parallel incident rays.
- C) The point where parallel rays of light diverge — This describes the behavior of a convex mirror (a diverging mirror), not a concave mirror. For a convex mirror, parallel rays appear to diverge from a virtual focal point behind the mirror.
- D) None of the above — Since option A correctly defines the focal point of a concave mirror, this option is incorrect.
5.
Which law of physics describes the relationship between the angle of incidence and the angle of reflection?
C.
Newton's first law of motion
ANSWER :
D. Law of reflection
Approach
The question asks to identify the law of physics that describes the relationship between the angle of incidence and the angle of reflection. We need to recall the definitions of the given laws and determine which one specifically addresses this phenomenon.
Step-by-step
- Understand the phenomenon: When light strikes a surface, it can be reflected. The angle at which it hits the surface (angle of incidence) and the angle at which it bounces off (angle of reflection) are related.
- Recall the Law of Reflection: This law states two main points: first, the incident ray, the reflected ray, and the normal to the surface at the point of incidence all lie in the same plane; second, the angle of incidence is equal to the angle of reflection. Mathematically, this is often written as \(\theta_i = \theta_r\), where \(\theta_i\) is the angle of incidence and \(\theta_r\) is the angle of reflection.
- Evaluate the options:
- Snell's Law: This law describes the relationship between the angles of incidence and refraction when light passes from one medium to another. It involves the refractive indices of the two media.
- Ohm's Law: This law relates voltage, current, and resistance in an electrical circuit.
- Newton's First Law of Motion: This law describes inertia, stating that an object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
- Law of Reflection: As discussed, this law directly defines the relationship between the angle of incidence and the angle of reflection.
- Conclusion: Based on the definitions, the Law of Reflection is the correct answer.
Correct Option: D) Law of reflection — This law states that the angle of incidence is equal to the angle of reflection. It directly describes the relationship between these two angles when light reflects off a surface.
Incorrect Options
- A) Snell's law — Snell's law (also known as the law of refraction) describes the relationship between the angle of incidence and the angle of refraction when light passes from one medium to another, not reflection. It is given by \(n_1 \sin \theta_1 = n_2 \sin \theta_2\).
- B) Ohm's law — Ohm's law relates voltage (\(V\)), current (\(I\)), and resistance (\(R\)) in an electrical circuit, expressed as \(V = IR\). It has no relation to light reflection.
- C) Newton's first law of motion — Newton's first law of motion deals with inertia and the state of motion of objects under the influence of forces. It is a fundamental principle of classical mechanics and is unrelated to the reflection of light.
6.
What happens to the speed of light when it travels from air into glass?
D.
Depends on the Wavelength
Approach
This question tests the understanding of how the speed of light changes when it passes from one medium to another, which is governed by the refractive index of the media involved.
Step-by-step
- Understanding Refractive Index: The refractive index (\(n\)) of a medium is a measure of how much the speed of light is reduced when it passes through that medium. It is defined as the ratio of the speed of light in a vacuum (\(c\)) to the speed of light in the medium (\(v\)).
\[ n = \frac{c}{v} \]
- Comparing Media: Air is considered an optically rarer medium, and its refractive index is approximately \(n_{\text{air}} \approx 1\). Glass is an optically denser medium, and its refractive index is typically around \(n_{\text{glass}} \approx 1.5\).
- Relating Speed to Refractive Index: From the formula, we can express the speed of light in a medium as:
\[ v = \frac{c}{n} \]
- Analyzing the Change: When light travels from air to glass, it moves from a medium with a lower refractive index (\(n_{\text{air}}\)) to a medium with a higher refractive index (\(n_{\text{glass}}\)). Since the speed of light in a medium (\(v\)) is inversely proportional to its refractive index (\(n\)), an increase in refractive index means a decrease in the speed of light.
- Conclusion: Therefore, when light travels from air into glass, its speed decreases.
Correct Option: B) Decreases
Incorrect Options
- A) Increases — This is incorrect. The speed of light decreases when it enters an optically denser medium like glass from an optically rarer medium like air.
- C) Remains the same — This is incorrect. The speed of light changes when it passes from one medium to another unless both media have the same refractive index.
- D) Depends on the Wavelength — While the refractive index itself can depend slightly on the wavelength (dispersion), the fundamental phenomenon is that the speed of light changes when entering a different medium. For a given wavelength, the speed will decrease when going from air to glass. The primary change is a decrease, not just a dependency without a clear direction.
