5B3 Geometric Optics

Question 1

Define:

optics

Answer 1

A branch of physics that deals with the generation, propagation, and detection of light.

It studies the properties and behavior of light, including how it interacts with matter.

Question 2

What are the two branches of optics?

Answer 2

• Geometrical optics
• Physical optics

These branches differ based on how light is treated: as rays in geometrical optics and as waves in physical optics.

Question 3

Define:

Geometrical optics

Answer 3

A branch of optics that studies the propagation of light in terms of rays, assuming light travels in straight lines unless reflected or refracted.

It is applicable when the wavelength of light is much smaller than the objects it interacts with, making wave effects like diffraction negligible.

Question 4

Define:

Light ray

Answer 4

It is an idealized model representing the path along which light energy travels.

Light rays are useful for predicting how light behaves in reflection, refraction, and image formation.

Question 5

True or False:

Light always travels in straight lines.

Answer 5

False

Light travels in straight lines in uniform media but bends when encountering different materials due to refraction.

Question 6

What are the three basic interactions of light in geometric optics?

Answer 6

• Reflection
• Refraction
• Absorption

Reflection → Follows the Law of Reflection.

Refraction → Described by Snell’s Law.

Absorption → Light converts into heat or energy.

Question 7

Define:

specular reflection

Answer 7

It occurs when light rays reflect off a smooth surface at equal angles, preserving the image.

Unlike diffuse reflection, which scatters light in many directions, specular reflection happens on surfaces like mirrors or calm water, producing clear images.

Question 8

What happens to light when it moves from one medium to another with a different density?

Answer 8

It changes speed and bends due to refraction.

If light moves to a denser medium (n1>n2), it slows down and bends toward the normal. If it moves to a less dense medium (n1<n2), it speeds up and bends away.

Question 9

What is Snell’s law?

Answer 9

n₁sin⁡θ₁=n₂sin⁡θ₂

It describes how light behaves when encountering different media. n₁ is the refractive index of the first medium, while n₂ is the refractive index of the second medium.

Question 10

Fill in the blank:

A _____ is a smooth surface that reflects light following the law of reflection.

Answer 10

mirror

The law of reflection states that the angle of incidence equals the angle of reflection: θ_i = θ_r.

Question 11

List two devices that rely on geometric optics principles.

Answer 11

1. Microscopes
2. Telescopes

Both devices use lenses and mirrors to manipulate light and magnify images.

Question 12

What property of light allows it to be focused by lenses and mirrors?

Answer 12

Light rays travel in predictable paths and follow specific reflection and refraction laws.

These properties allow optical systems to form sharp images, used in cameras, glasses, and optical instruments.

Question 13

Define:

Plane mirror

Answer 13

Flat reflective surface that forms an upright, virtual, and same-size image as the object.

The image distance in a plane mirror is equal to the object distance, and the image is laterally inverted.

Question 14

Fill in the blank:

A ______ mirror has a reflective surface that curves outward, like the exterior of a sphere.

Answer 14

Convex

Convex mirrors diverge light rays and provide a wider field of view, making them useful for security mirrors and vehicle side mirrors.

Question 15

True or False:

A concave mirror has a reflective surface that curves inward, like the inside of a sphere.

Answer 15

True

Concave mirrors converge light rays and are commonly used in telescopes, shaving mirrors, and headlights due to their ability to focus light.

Question 16

Fill in the blank:

A _____ is a transparent optical device that refracts light rays to converge or diverge, forming an image.

Answer 16

lens

Lenses are classified as converging (convex) or diverging (concave), depending on their shape and how they affect light rays.

Question 17

What is the main difference between converging and diverging lenses?

Answer 17

Converging lenses focus light rays, while diverging lenses spread them apart.

This difference affects how images are formed: Converging lenses can form real or virtual images, but diverging lenses only form virtual images.

Question 18

What is a meniscus lens?

Answer 18

A lens with one convex and one concave surface that can converge or diverge light, depending on its curvature.

Positive meniscus lenses converge light; negative meniscus lenses diverge it.

Question 19

Fill in the blank:

A ______-______ lens is curved on one side and flat on the other.

Answer 19

Plano-convex or plano-concave

Plano-convex lenses focus light, while plano-concave lenses spread light out.

Question 20

True or False:

A biconcave lens is thicker at the center than at the edges.

Answer 20

False

A biconcave lens is thinner at the center and diverges light. Biconcave lenses are used in applications like correcting myopia (nearsightedness).

Question 21

Define:

Focal point

of a lens or mirror

Answer 21

The point where parallel light rays converge (or appear to diverge from).

For a concave mirror or convex lens, light rays actually converge at the focal point. For a convex mirror or concave lens, rays appear to diverge from it.

Question 22

What is ray tracing in geometric optics?

Answer 22

A method for determining how light interacts with surfaces using straight-line approximations.

Ray tracing helps predict image formation in lenses and mirrors by following the paths of a few representative rays.

Question 23

Why is a ray diagram useful in optics?

Answer 23

It helps predict the position, size, orientation, and type (real or virtual) of an image formed by mirrors or lenses.

Ray diagrams use key principle rays to graphically determine how light interacts with optical elements like lenses and mirrors.

Question 24

Explain the difference between real and virtual images.

Answer 24

Real images can be projected onto a screen, while virtual images cannot.

Real images form where light rays converge, while virtual images appear to originate from a point where rays seem to diverge.

Question 25

List the three principal rays used in **ray tracing for lenses**.

Answer 25

1. Parallel ray 2. Focal ray 3. Chief ray ## Footnote These rays help locate an image: the **parallel ray** refracts through the focal point, the **focal ray** passes through the focal point before becoming parallel, and the **chief ray** passes through the lens center, continuing undeviated.

Question 26

A parallel ray strikes a **concave mirror**. Through what point does it reflect?

Answer 26

It reflects through the **focal point**. ## Footnote This is a key rule in ray tracing: rays parallel to the principal axis converge at the mirror's focal point after reflection.

Question 27

# Fill in the blank: In **convex lens ray tracing**, a ray passing through the \_\_\_\_\_\_ \_\_\_\_\_ remains undeviated.

Answer 27

Optical center ## Footnote Since the optical center is where the lens is thinnest, refraction does not significantly alter the ray’s path.

Question 28

Explain why **at least two rays** are needed to locate an image in ray tracing.

Answer 28

An **image forms where multiple light rays converge** or appear to diverge from. ## Footnote Using two or more rays ensures that the image location is consistent with optical principles.

Question 29

What happens when a ray **passes through the focal point before entering a convex lens**?

Answer 29

It **exits the lens parallel** to the principal axis. ## Footnote This is the reverse of what happens to parallel rays entering the lens—ray paths are symmetrical in optics.

Question 30

What **properties** of a lens or mirror determine the **direction** of refracted or reflected rays?

Answer 30

* The curvature * The focal length ## Footnote **Curvature** affects how light bends or reflects, while the **focal length** determines how strongly the system converges or diverges light.

Question 31

What is the purpose of the **principal axis** in ray tracing?

Answer 31

It serves as a **reference line** for drawing and analyzing light rays. ## Footnote The principal axis passes through the optical center and focal points of a lens or mirror, providing a symmetrical framework for ray diagrams.

Question 32

# Fill in the blank: If the object is located at the **focal point of a converging lens**, the image is formed at \_\_\_\_\_\_.

Answer 32

Infinity ## Footnote At the focal point, the refracted rays emerge parallel, meaning they **never converge** to form a visible image.

Question 33

What equation relates **focal length**, **image distance**, and **object distance**?

Answer 33

1/f=1/dₒ+1/dᵢ ## Footnote f is focal length, dₒ is object distance, and dᵢ is image distance. This applies to both lenses and mirrors.

Question 34

If an object is placed **40 cm** away from a **concave mirror** with a focal length of **25 cm**, what is the image distance?

Answer 34

66.7 cm ## Footnote Positive, meaning the image is real and inverted. Use the equation: 1/f=1/dₒ+1/dᵢ. Where f=25 cm, and dₒ=40 cm.

Question 35

# Define: Magnification | in optics

Answer 35

**Ratio** of the image size to the object size. ## Footnote It indicates how much larger or smaller the image appears, measured with the equation M=hi/ho. M > 1: Enlarged image M < 1: Reduced image M negative: Inverted image

Question 36

What does a **negative magnification (M)** indicate?

Answer 36

The image is **inverted**. ## Footnote Magnification is given by **M=-dᵢ/dₒ**. A positive value means upright, while a negative value means inverted.

Question 37

A **convex mirror** always produces what kind of image?

Answer 37

Virtual, upright, and reduced. ## Footnote Since convex mirrors cause rays to diverge, they never form real images—only virtual ones that appear behind the mirror.

Question 38

What happens to the **image size** when the object moves closer to a **converging lens**?

Answer 38

It becomes **larger**. ## Footnote As the object nears the focal point, the image distance increases, leading to greater magnification.

Question 39

Why do **concave lenses** always **form smaller images**?

Answer 39

Because they cause light rays to diverge, forming only virtual, reduced images. ## Footnote Unlike convex lenses, concave lenses spread light apart, making objects appear **smaller and closer**.

Question 40

What is the significance of the **negative focal length** in a convex mirror?

Answer 40

Indicates that the **focal point is behind the mirror**. ## Footnote This is a characteristic of convex mirrors.

Question 41

An object is placed **beyond twice the focal length** of a convex lens. What can you say about the image?

Answer 41

The image is real, inverted, and smaller. ## Footnote When dₒ > 2f, the image forms between f and 2f on the opposite side of the lens.

Question 42

What is the purpose of a **magnifying glass**?

Answer 42

To produce a **larger, virtual, upright** image of a close object. ## Footnote It is a **converging lens** held within the focal length of the object.

Question 43

Why do **microscopes** use two lenses?

Answer 43

To achieve **greater magnification** by combining the effects of the objective and eyepiece lenses. ## Footnote The objective lens forms a real image, which is then magnified by the eyepiece lens.

Question 44

What **optical principle** allows **telescopes** to collect distant light?

Answer 44

**Refraction (lenses) or reflection (mirrors**) to focus light from distant objects. ## Footnote Refracting telescopes use lenses, while reflecting telescopes use concave mirrors.

Question 45

What are the two **main types of telescopes**?

Answer 45

1. Refracting telescopes 2. Reflecting telescopes ## Footnote Refracting telescopes → Use lenses to bend & focus light. Reflecting telescopes → Use concave mirrors to gather light.

Question 46

# Define: Chromatic aberration

Answer 46

A **distortion that occurs in refracting telescopes** due to varying wavelengths of light being refracted at different angles. ## Footnote It results in images appearing as separate-colored overlays.

Question 47

# True or False: A **microscope** forms a real image for the observer.

Answer 47

False ## Footnote The final image seen through the eyepiece is virtual and magnified. The objective lens first forms a real, inverted image, which the eyepiece magnifies further.

Question 48

How does a **prism** separate white light?

Answer 48

Through **dispersion**, where different wavelengths refract at different angles. ## Footnote Shorter wavelengths (blue, violet) bend more than longer wavelengths (red, orange).

Question 49

What is the purpose of **prisms in binoculars**?

Answer 49

To **invert and shorten the light path** while maintaining a clear image. ## Footnote Binoculars use Porro or roof prisms to correct image orientation.

Question 50

How does a camera lens **focus** light?

Answer 50

By adjusting the **focal length** to form a clear image on the sensor. ## Footnote Autofocus systems move lens elements to optimize sharpness.

Question 51

Why do telescopes have a **larger objective lens or mirror**?

Answer 51

To **collect more light**, improving image brightness and resolution. ## Footnote A larger **aperture** results in **better detail and visibility** in low-light conditions.

Question 52

What is a **periscope**, and how does it work?

Answer 52

A device using **two plane mirrors** to allow vision over obstacles. ## Footnote Common in **submarines and armored vehicles** for safe observation.

Question 53

# True or False: **Periscopes** use **specular reflection** in mirrors to allow viewing over obstacles or around corners.

Answer 53

True ## Footnote Periscopes use two plane mirrors placed at 45° angles to reflect light in a predictable way, maintaining the image’s orientation. They are commonly used in submarines and tanks.

Question 54

Why do **microscopes** have an adjustable stage?

Answer 54

To **control image focus and depth of field** by adjusting the object’s position. ## Footnote Precise adjustments allow sharp viewing at higher **magnifications**.

Question 55

How do you calculate the total **magnification** of a compound **microscope**?

Answer 55

Multiply the ocular lens magnification (10x) by the objective lens magnification (4x, 10x, 40x, or 100x). ## Footnote For example, using a 4x objective lens gives 40x total magnification. Remember the formula: *Total Magnification=Ocular × Objective*

Question 56

How does the **human eye** form images?

Answer 56

The eye uses a **convex lens** to refract light onto the retina, forming a real and inverted image. ## Footnote The brain processes this image and perceives it as upright. Glasses and contact lenses correct vision by adjusting how light is refracted before entering the eye.

Question 57

What causes **nearsightedness**?

Answer 57

**Light focuses too early**, in front of the retina. ## Footnote Results in a blurry image for the brain, known as myopia.

Question 58

What causes **astigmatism**, and how does it affect vision?

Answer 58

It happens when the **cornea or lens has an uneven curvature**, causing blurry or distorted vision. ## Footnote A normal cornea is spherical, but in astigmatism, it has an uneven shape, leading to light focusing at multiple points on the retina instead of a single point. This condition is often corrected with cylindrical lenses.