Samacheer Kalvi 10th Science Chapter 22 Short Answer Questions — Visual Communication with Answers

The human eye is a spherical organ of about 2.5 cm diameter, protected by the bony socket (orbit) and eyelids.

Note: In very dim light, only rods function — that's why we can't see colours well in the dark, and why objects look clearer slightly off-centre (more rods there).

• Problem: Cannot see distant objects clearly; image of distant objects forms in front of retina
• Causes: Eyeball too long (antero-posterior diameter increased), or eye lens too curved
• Correction: Concave (diverging) lens — diverges light rays before they enter the eye so they focus exactly on retina

• Problem: Cannot see nearby objects clearly; image of near objects forms behind retina
• Causes: Eyeball too short, or eye lens too flat, or ciliary muscles too weak
• Correction: Convex (converging) lens — converges light rays so they focus on retina

• Problem: Age-related (usually after 40) loss of ability to focus on nearby objects
• Cause: Eye lens loses elasticity; ciliary muscles weaken
• Correction: Bifocal lenses — upper part is concave (for distance), lower part is convex (for near vision)

• Problem: Blurred or distorted vision at all distances; horizontal and vertical lines cannot be in focus simultaneously
• Cause: Uneven curvature of the cornea or lens (not perfectly spherical)
• Correction: Cylindrical lenses — designed to correct uneven curvature

Structure of a simple camera:

Persistence of vision is the phenomenon where the retina retains the impression of an image for about 1/16th of a second (0.0625 seconds) after the object has been removed from view.

Cause: The photoreceptors (rods and cones) continue to send nerve signals to the brain for a brief time after the stimulus is gone. The brain perceives this lingering image as still present.

Applications:

• Still photographs (frames) are projected at 24 frames per second (fps)
• Due to persistence of vision, each frame appears to blend with the next
• The brain perceives this sequence as continuous motion
• This is why movies appear as smooth motion rather than flickering individual photographs

• Images are refreshed at 25 fps (PAL system) or 30 fps (NTSC system)
• Same principle applies

• Traditional animation draws slightly different images on each frame
• At 24+ fps, the brain combines them as smooth movement

• Phenakistoscope (1832) and Zoetrope — early animation devices based on this principle
• Thaumatrope — spinning disc with images on both sides that merge

Threshold: Below ~16 fps, the brain perceives flickering rather than continuous motion (the "flicker fusion frequency").

Film photography uses light-sensitive chemical coatings on film to record images.

Photographic film composition:

• Plastic base coated with gelatin emulsion containing silver halide crystals (mainly silver bromide AgBr, also AgCl and AgI)

Formation of a latent image:

• When light strikes silver halide crystals, a photochemical reaction occurs:
• 2AgBr + light → 2Ag + Br₂ (photolysis — decomposition by light)
• The silver atoms cluster to form the latent image (invisible at this stage)
• Areas that receive more light have more silver atoms formed

Development:

• The film is treated with a developer (reducing agent like hydroquinone)
• Developer reduces unexposed AgBr to silver → the latent image becomes a visible dark negative image
• Areas that received more light appear darker (more silver metal)

Fixing:

• Sodium thiosulphate (hypo) dissolves remaining unexposed silver halide
• This makes the image permanent and stable in normal light

Printing:

• Light is shone through the negative onto photographic paper
• Same chemistry produces a positive print

Digital photography replaced this process using CCD/CMOS sensors that convert light directly to electrical signals.