NCERT Solutions: Chapter10 Light: Mirrors and Lenses (class 8 Science)

Yes, we can make mirrors that produce enlarged or diminished images:

• Concave mirrors: Can form both enlarged and diminished images (as well as real or virtual images), depending on the distance of the object from the reflecting surface.
• Convex mirrors: Always produce virtual, erect, and diminished (smaller) images of objects placed in front of them, regardless of distance.

The warning "Objects in mirror are closer than they appear" is written on the side-view mirrors of vehicles because these mirrors are convex mirrors:

• Convex mirrors curve outwards, forming diminished (smaller) images of vehicles coming from behind.
• Because our brain naturally associates smaller image sizes with greater distances, an approaching vehicle appears to be farther away than it actually is.
• The convex mirror provides a much wider field of view to the driver, but the cautionary text alerts the driver to avoid misjudging distance when changing lanes or braking.

The curved line on reading glasses is due to the convex shape of the lens or the boundary of a bifocal segment:

• These lenses are thicker in the middle and thinner at the edges (convex lenses), which causes incident light rays to converge (bend inward).
• This convergence increases refractive power, helping focus nearby light rays properly on the retina to correct near vision problems like presbyopia or hypermetropia.

Curious and engaging questions from the chapter include:

• How does a magnifying glass make tiny text look significantly bigger?
• Why do the inner and outer curved surfaces of a shiny stainless steel spoon show completely different images?
• Can the crystalline lenses inside our human eyes change their curvature and shape?
• What happens when you use a large concave mirror to focus intense sunlight onto a point?

Yes, image formation by all mirrors is strictly governed by the Laws of Reflection:

1. First Law: The angle of incidence is always equal to the angle of reflection ($\angle i = \angle r$).
2. Second Law: The incident ray, the reflected ray, and the normal to the reflecting surface at the point of incidence all lie in the same plane.

Yes, the laws of reflection apply universally to spherical mirrors (concave and convex) as well as plane mirrors. At every point on a curved mirror surface, the normal is along the radius connecting that point to the center of curvature ($C$). The light ray reflects such that the angle of incidence equals the angle of reflection ($\angle i = \angle r$) and all three lines lie in the same plane.

When an object is viewed through a lens, several distinct optical changes are observed:

• Change in Size: The image can appear enlarged (magnified), same size, or diminished (smaller) depending on lens type and distance.
• Change in Orientation: The image may appear upright (erect) or flipped upside down (inverted).
• Image Nature: The image can be virtual (viewed through the lens) or real (projected onto a screen). Specifically, concave lenses always form diminished, virtual, and erect images.

Yes, lenses bend light beams through refraction:

• Convex Lens (Converging Lens): Thicker at the center than at the edges. It converges parallel incoming light rays to a single focal point ($F$).
• Concave Lens (Diverging Lens): Thinner at the center than at the edges. It diverges (spreads outward) parallel incoming light rays so they appear to originate from a virtual focus behind the lens.

Yes. A convex lens acts as a converging lens. When held facing the Sun, it concentrates the parallel rays of sunlight and their associated radiant thermal energy into a sharp, intense focal spot. The heat concentrated at this tiny focal point can quickly reach the ignition temperature of dry paper, causing it to smoke and burn.

Lenses are widely utilized across everyday devices and advanced scientific optical instruments:

• Spectacles and Contact Lenses: Correct vision defects such as myopia (concave lenses) and hypermetropia/presbyopia (convex lenses).
• Magnifying Glasses: Single convex lenses used to read fine print or inspect jewelry.
• Cameras and Smart Phones: Multi-lens systems used to focus sharp images onto digital sensors.
• Microscopes and Telescopes: Compound lens systems used to magnify microscopic cells or view distant planets and stars.
• Projectors and Door Spyholes: Projectors use convex lenses to display enlarged images on screens; door peepholes use concave lenses to give a wide-angle view of visitors.

Correct Answer: (ii) 50°

Step-by-Step Calculation:

• Given angle of incidence: $\angle i = 40^\circ$ (angle between incident ray and normal).
• According to the Law of Reflection: Angle of reflection $\angle r = \angle i = 40^\circ$ (angle between reflected ray and normal).
• The normal is perpendicular to the mirror surface ($90^\circ$).
• Therefore, the angle made by the reflected ray with the mirror surface is: $$\text{Angle with mirror} = 90^\circ - \angle r = 90^\circ - 40^\circ = 50^\circ$$

• Case (i): Ray falls along the normal:
  $\text{Angle of incidence } (\angle i) = 0^\circ$.
  $\text{Angle of reflection } (\angle r) = 0^\circ$.
  Result: The reflected ray retraces its original path straight back along the normal line.
• Case (ii): Tilted mirror with ray along normal:
  Because the ray is perpendicular to the tilted surface, $\angle i = 0^\circ$.
  $\text{Angle of reflection } (\angle r) = 0^\circ$.
  Result: The ray again retraces its path perpendicular to the tilted surface.
• Case (iii): Ray falls at 20° from the normal:
  $\text{Angle of incidence } (\angle i) = 20^\circ$.
  $\text{Angle of reflection } (\angle r) = 20^\circ$.
  Result: The reflected ray makes an angle of $20^\circ$ with the normal on the opposite side of the normal.

Correct Answer: (ii) Angle of incidence is 0°

Explanation: The angle of incidence is defined as the angle between the incident ray and the normal. When light strikes a mirror perpendicularly (along the normal line), the incident ray coincides with the normal. Therefore, the angle of incidence is $0^\circ$, and by the law of reflection, the angle of reflection is also $0^\circ$.

Observation & Optical Analysis:

A convex mirror always forms a virtual, erect, and diminished (smaller) image. As the woman walks closer to the mirror from a distance:

• Her image remains upright (erect) at all times.
• The size of the erect image gradually increases as she gets closer (though it always remains smaller than her real physical size).

A magnifying glass uses a convex lens (converging lens):

• Held close to text (within focal length $f$): The text appears clear, erect, and significantly magnified.
• Moving farther away: As the lens moves past its focal point, the image blurs, flips upside down (becomes inverted), and gradually decreases in size.

Correct Answer: (i) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.

Explanation: Convex mirrors curve outwards and diverge reflected light rays. This enables them to capture a significantly wider field of view compared to plane mirrors of the same size. Although the erect images formed are diminished, the wide field of view lets drivers monitor multiple lanes of trailing traffic, drastically enhancing road safety.

Correct Answer: (ii) Figure (a) indicates a convex mirror and Figure (b) indicates a concave mirror.

Explanation: In Figure (a), the image ($I$) is upright and distinctly smaller (diminished) than the object ($O$), which is characteristic of a convex mirror. In Figure (b), the image ($I$) is upright and visibly larger (magnified/enlarged) than the object ($O$), which is characteristic of a concave mirror when an object is held close to it.

The pencil appears bent or broken at the water-air interface, and the submerged part looks slightly thicker and laterally shifted. This optical illusion is caused by the refraction of light.

Why it happens:

• Change in Medium & Speed: Light travels at different speeds in different optical media. Air is an optically rarer medium (light travels faster), while water is an optically denser medium (light travels slower).
• Bending of Light Rays: As light rays from the submerged pencil exit the water into the air on their way to our eyes, they speed up and bend away from the normal.
• Apparent Position: The human brain perceives light as traveling in straight lines. Projecting these bent rays backward creates a virtual image of the submerged pencil that is displaced from its actual physical position.

Both dentists and ENT specialists utilize concave mirrors:

• Dentists: Use small concave mouth mirrors placed close to teeth. When held within the mirror's focal length, the concave surface creates an upright, magnified virtual image of small cavities and tooth structures.
• ENT Specialists: Use concave head-mirrors to reflect and concentrate a beam of light from an external lamp directly into narrow cavities of the ear, nose, or throat, illuminating hidden areas clearly.

Working Principle: A concave reflecting mirror converges parallel sunlight rays to a concentrated focal spot. A black-painted metal cooking container placed at this focal spot absorbs maximum radiant heat to cook food without electricity or gas fuel.

Using digital simulation tools (such as PhET Interactive Simulations - Geometric Optics):

• Concave Mirror: Bringing an object very close to the mirror creates a large, upright, virtual image. Moving it beyond the focal point causes the image to invert (flip upside down) and become real.
• Convex Mirror: At every distance, the image remains upright, virtual, and diminished, illustrating why it provides a wide panoramic view in vehicle side mirrors.
• Convex Lens: When an object is within focal length $f$, it forms an enlarged, upright virtual image (magnifying glass effect). Beyond $f$, it forms an inverted real image that can be focused on a screen (camera/projector effect).
• Concave Lens: Spreads light rays apart, consistently forming virtual, erect, and diminished images regardless of object position (used in glasses for nearsightedness).