1. Ray optics mainly describes light by treating it as travelling along what kind of path in a uniform medium?
ⓐ. Straight-line rays
ⓑ. Circular paths around the source
ⓒ. Random curved paths
ⓓ. Stationary lines with no direction
Correct Answer: Straight-line rays
Explanation: Ray optics uses the idea that light can be represented by rays. A ray shows the direction in which light travels. In a uniform transparent medium, these rays are taken to move along straight lines. This simple model helps in drawing mirror and lens diagrams without first using the full wave nature of light. Curved paths are considered only in special situations such as a medium whose refractive index changes from place to place. At the beginning of ray optics, the straight-ray model is the working picture used to locate images.
2. In a ray diagram for a mirror or lens, the main purpose of drawing rays from an object point is to locate
ⓐ. the mass of the object
ⓑ. the colour absorbed by the object
ⓒ. the temperature of the optical device
ⓓ. the image position
Correct Answer: the image position
Explanation: In ray optics, rays are drawn from an object point and then followed after reflection or refraction. If the actual reflected or refracted rays meet, or if their backward extensions appear to meet, that point gives the image position. The diagram does not measure mass or temperature, because those are not optical image-location quantities. Colour may matter in dispersion, but it is not the basic purpose of an image-forming ray diagram. Image formation in ray optics is mainly a geometry problem involving the directions of rays.
3. Which statement best separates a real image from a virtual image?
ⓐ. A real image is always larger, while a virtual image is always smaller
ⓑ. A real image is always erect, while a virtual image is always inverted
ⓒ. A real image can be obtained on a screen; a virtual image cannot
ⓓ. A real image is formed only by lenses, while a virtual image is formed only by mirrors
Correct Answer: A real image can be obtained on a screen; a virtual image cannot
Explanation: A real image is formed where light rays actually meet after reflection or refraction. Because light really reaches that position, the image can be caught on a screen placed there. A virtual image is formed where rays only appear to come from when traced backward. Such an image can be seen by the eye, but it cannot be projected directly on a screen. Size and orientation depend on the optical device and object position, so they cannot be used as the general distinction.
4. A plane mirror shows your face behind the mirror surface. This image is called virtual because
ⓐ. light rays actually pass through the space behind the mirror
ⓑ. the image has no height
ⓒ. the mirror changes the speed of light behind it
ⓓ. reflected rays appear to meet behind the mirror
Correct Answer: reflected rays appear to meet behind the mirror
Explanation: In a plane mirror, reflected rays come back into the region in front of the mirror. They do not actually pass behind the mirror and meet there. When these reflected rays are extended backward, they appear to come from a point behind the mirror. That apparent point gives the virtual image. The image has a definite apparent size and position, but the light does not physically converge behind the mirror.
5. An optical device is useful in ray optics when its action can be understood by following the change in direction of light rays. A suitable example is
ⓐ. a thermometer measuring body temperature
ⓑ. a spring balance measuring weight
ⓒ. a convex lens imaging a candle
ⓓ. a tuning fork producing sound
Correct Answer: a convex lens imaging a candle
Explanation: A convex lens changes the direction of light rays by refraction and can form an image of an object. Its image formation can be studied by drawing a few standard rays from the object. A thermometer, spring balance, and tuning fork involve other physical effects, not ray-based image formation. Ray optics is especially suited to mirrors, lenses, prisms, spectacles, microscopes, and telescopes. The common link in these devices is the controlled change in ray direction.
6. The branch of optics used for ordinary mirror and lens image diagrams relies most directly on
ⓐ. chemical changes in glass
ⓑ. heating of the optical surface
ⓒ. geometrical ray construction
ⓓ. magnetic force on light rays
Correct Answer: geometrical ray construction
Explanation: Ray optics is also called geometrical optics because many results are obtained using geometry. Rays are drawn, angles are measured, and their intersections or apparent intersections are used to locate images. The basic constructions do not require chemical changes in the material. Heating and magnetic force are not the usual reasons for image formation by ordinary mirrors and lenses. This is why ideas such as normal, angle of incidence, focus, and principal axis become central to geometrical image formation.
7. In ray optics, a straight line with an arrow is drawn to represent a light ray. The arrow mainly shows
ⓐ. the size of the source
ⓑ. the direction of propagation of light
ⓒ. the speed value \(3\times 10^8\,\text{m s}^{-1}\)
ⓓ. the brightness of the image
Correct Answer: the direction of propagation of light
Explanation: A ray is a directed line used to show the path followed by light. The arrow on the ray tells the direction in which light is travelling. The length of the drawn line does not by itself give the size of the source or the brightness of the image. The speed of light may be needed in refractive-index discussions, but it is not what the arrow directly represents. In ray diagrams, direction is more important than the drawn length of the ray.
8. A screen is placed at the position where reflected rays from a concave mirror actually meet. The bright image on the screen is an example of
ⓐ. a real image
ⓑ. a virtual image
ⓒ. lateral inversion only
ⓓ. apparent bending without image formation
Correct Answer: a real image
Explanation: When reflected rays actually converge at a point, light energy reaches that point. A screen placed there receives the rays and shows the image. This is the defining feature of a real image. A virtual image is different because the rays only seem to come from a point and cannot be collected on a screen at that apparent position. The screen test is one of the simplest ways to separate real and virtual images.
9. A person sees an image in a plane mirror, but a screen placed behind the mirror does not catch that image. The best interpretation is that the image is
ⓐ. real because the image is visible to the eye
ⓑ. real because the mirror has a polished surface
ⓒ. virtual, with rays only appearing to meet behind the mirror
ⓓ. absent because a visible image must always form on a screen
Correct Answer: virtual, with rays only appearing to meet behind the mirror
Explanation: Visibility to the eye does not automatically mean that an image is real. In a plane mirror, the eye receives reflected rays from the front side of the mirror and traces them backward. The rays seem to come from behind the mirror, so the image appears there. A screen behind the mirror cannot catch the image because the reflected rays do not actually go there and converge. The word virtual describes this apparent meeting of rays, not an unreal visual experience.
10. The use of geometry in ray optics is most natural because many image positions are found from
ⓐ. changes in the mass of the object
ⓑ. ray intersections or their backward extensions
ⓒ. changes in gravitational field near the mirror
ⓓ. chemical reactions at the optical surface
Correct Answer: ray intersections or their backward extensions
Explanation: Ray optics follows the paths of rays after reflection or refraction. If the rays themselves meet, the image is real. If the backward extensions of rays meet, the image is virtual. Both cases can be handled by geometrical drawing and angle relations. This is why image formation by mirrors and lenses is often introduced through ray diagrams before detailed formula work.
11. In a basic ray-optics description of spectacles, the lenses are used mainly to
ⓐ. change the colour of light entering the eye
ⓑ. convert light into sound
ⓒ. stop all rays from reaching the eye
ⓓ. refract light before it enters the eye
Correct Answer: refract light before it enters the eye
Explanation: Spectacle lenses work by refracting light before it enters the eye. Refraction changes the direction of rays so that the eye can focus them properly on the retina. The lens does not add mass to light or convert it into sound. It also does not block all rays, because then vision would not be possible. Even before detailed eye-defect formulas, the ray-optics idea is that controlled bending of rays helps form a clear image.
12. A microscope and a telescope are both included in ray optics because their working can be started by studying
ⓐ. how their bodies are painted
ⓑ. how their lenses or mirrors form images
ⓒ. how sound travels through their tubes
ⓓ. how their metal parts expand on heating
Correct Answer: how their lenses or mirrors form images
Explanation: A microscope and a telescope use lenses or mirrors to control the paths of light rays. Their image formation can be introduced by following rays through the objective and eyepiece. The paint, sound in the tube, and thermal expansion of parts are not the central optical principles. These instruments differ in purpose, but both depend on forming and viewing images. Ray optics provides the first working model for understanding their image paths and magnification.
13. In the usual notation of image formation by a mirror or lens, the symbol \(u\) represents
ⓐ. image distance
ⓑ. focal length
ⓒ. linear magnification
ⓓ. object distance
Correct Answer: object distance
Explanation: The symbol \(u\) is used for the object distance measured from the pole of a mirror or the optical centre of a lens, depending on the device. The image distance is usually denoted by \(v\). The focal length is denoted by \(f\), and magnification is denoted by \(m\). These symbols become especially important when sign convention is used in formula-based problems. Treating \(u\), \(v\), and \(f\) as interchangeable is a common source of wrong substitutions in ray optics.
14. A spherical mirror has radius of curvature \(R\) and focal length \(f\). The most suitable unit pair for these two quantities is
ⓐ. same length unit for both
ⓑ. \(R\) in \(m\), \(f\) in \(\text{dioptre}\)
ⓒ. \(R\) in \(\text{dioptre}\), \(f\) in \(m\)
ⓓ. \(R\) in degree, \(f\) in \(cm\)
Correct Answer: same length unit for both
Explanation: Radius of curvature \(R\) and focal length \(f\) are both lengths. They may be written in \(m\) or in \(cm\), but the same length unit should be used consistently in a formula. Dioptre is the unit of lens power, not of radius or focal length. Degree is used for angles such as angle of incidence or angle of refraction. Since \(f=\frac{R}{2}\) for a spherical mirror under the usual approximation, both sides of the relation must have the same length unit.
15. The optical quantity among the following that has no unit is
ⓐ. focal length \(f\)
ⓑ. refractive index \(n\)
ⓒ. radius of curvature \(R\)
ⓓ. object distance \(u\)
Correct Answer: refractive index \(n\)
Explanation: Refractive index \(n\) is a ratio of two speeds, such as \(n=\frac{c}{v}\). Since both \(c\) and \(v\) have the same unit, the units cancel. Therefore \(n\) is dimensionless. Focal length \(f\), radius of curvature \(R\), and object distance \(u\) are all lengths and need length units such as \(m\) or \(cm\). A value like \(n=1.5\) is meaningful without attaching \(m\), \(cm\), or \(\text{dioptre}\) to it.
16. A thin convex lens has focal length \(0.50\,m\). Its power is
ⓐ. \(+0.50\,\text{dioptre}\)
ⓑ. \(-2.0\,\text{dioptre}\)
ⓒ. \(+50\,\text{dioptre}\)
ⓓ. \(+2.0\,\text{dioptre}\)
Correct Answer: \(+2.0\,\text{dioptre}\)
Explanation: \( \textbf{Given data:} \)
Focal length of the convex lens is \(f=0.50\,m\).
\( \textbf{Required quantity:} \)
Power \(P\) of the lens.
The relation between power and focal length is
\[
P=\frac{1}{f}
\]
This formula requires \(f\) in \(m\), and it is already given in \(m\).
Substituting the value,
\[
P=\frac{1}{0.50}=2.0\,\text{dioptre}
\]
A convex lens has positive focal length in the usual lens sign convention, so its power is positive.
\( \textbf{Final answer:} \) \(+2.0\,\text{dioptre}\); using \(50\,cm\) directly in \(P=\frac{1}{f}\) would give a wrong numerical scale.
17. The angle of incidence is measured between the incident ray and the
ⓐ. normal at the point of incidence
ⓑ. reflecting or refracting surface
ⓒ. principal axis only
ⓓ. reflected ray only
Correct Answer: normal at the point of incidence
Explanation: The angle of incidence is always measured from the normal drawn at the point where the ray strikes the surface. It is not measured from the surface itself. The normal is perpendicular to the surface at that point, so measuring from the surface would give the complementary angle. The same idea is used for the angle of reflection and the angle of refraction. This convention keeps the laws of reflection and refraction written in a simple and standard form.
18. In the usual sign language for image height, an image formed below the principal axis has height
ⓐ. negative
ⓑ. positive
ⓒ. zero
ⓓ. always equal to object height
Correct Answer: negative
Explanation: Heights in ray optics are measured with respect to the principal axis. A height above the principal axis is taken as positive, while a height below the principal axis is taken as negative. This sign helps to distinguish erect and inverted images through magnification. An inverted real image often has negative image height when the object height is taken positive. The sign of height tells orientation, not the physical absence of the image.
19. A notebook contains the following entries for basic optical symbols:
| Row | Symbol | Meaning | Usual unit |
|---|
| P | \(v\) | image distance | \(m\) or \(cm\) |
| Q | \(n\) | refractive index | \(\text{dioptre}\) |
| R | \(i\) | angle of incidence | \(m\) |
| S | \(P\) | power of lens | \(\text{dioptre}\) |
The fully suitable rows are
ⓐ. Q and R only
ⓑ. P, Q, and S only
ⓒ. P and S only
ⓓ. R and S only
Correct Answer: P and S only
Explanation: Row P is suitable because \(v\) denotes image distance, and image distance is a length measured in \(m\) or \(cm\). Row S is also suitable because \(P\) denotes lens power, whose unit is \(\text{dioptre}\). Row Q is not suitable because refractive index \(n\) is dimensionless, not measured in \(\text{dioptre}\). Row R is not suitable because angle of incidence \(i\) is measured in degree or radian, not in \(m\). This table separates length quantities, angle quantities, dimensionless ratios, and lens power.
20. In the New Cartesian sign convention commonly used for mirrors and lenses, if incident light travels from left to right, distances measured to the left of the origin are generally
ⓐ. positive
ⓑ. negative
ⓒ. always zero
ⓓ. ignored in formula use
Correct Answer: negative
Explanation: The origin is taken at the pole of a mirror or the optical centre of a lens. The direction of incident light is usually taken as the positive direction along the principal axis. If incident light travels from left to right, distances measured to the right are positive and distances measured to the left are negative. For a real object placed in front of a mirror or lens on the left side, the object distance \(u\) is therefore usually negative. This sign convention allows image nature and position to come out from the same algebraic formula.
