What is a phenomenon where number of images formed as the angle between two mirror decreases

Question 8 Exercise 7B

Answer:

(a) The number of images formed when an object is placed between the two plane mirrors at an angle of 90°, is 3. Three images are formed.

We know that two mirrors kept perpendicular to each other, produce three images for an object that is placed in between them.

i.e, the angle between two mirrors is 60°, n=360°/90° = 4, number of images = n-1 = 4-1 = 3.

(b) The number of images formed if an object is placed between two plane mirrors with an angle of 60°, is five. Five images are formed:

i.e, the angle between two mirrors is 60°, n=360°/60° = 6, number of images = n-1 = 6-1 = 5.

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All in One - SCIENCE CBSE Class 8>Light>Chapter Practice>Q 3

(a)first increases then decreases

(b)first decreases then increases

22.22% students answered this correctly

1. Laws of Reflection:

(i) Light is an invisible energy which causes the sensation of vision.

(ii) Light energy always travels in straight lines.

(iii) Incident ray: A ray of light which travels towards a mirror or some other medium.

(iv) Angle of incidence: The angle which the incident ray makes with normal at the point of incidence.

(v) Angle of reflection: The angle which the reflected ray makes with normal at the point of incidence.

(vi) There are two laws of reflection:

(a) incident ray, reflected ray and the normal lie in the same plane at the point of incidence

(b) angle of incidence is always equal to the angle of reflection.

2. Regular and Diffused Reflection:

(i) Regular reflection: The phenomenon due to which a parallel beam of light is reflected as a parallel beam in some other direction.

(ii) Diffused reflection: When a parallel beam of light on striking some rough surface gets reflected in different directions.

(iii) Mirror: A highly polished surface from which reflection takes place.

(iv) Real image: An inverted image which can be projected on the screen.

(v) Virtual image: An erect image which cannot be taken on the screen.

(vi) When the left side of the object appears as the right side of an image or vice versa, the image is said to be laterally inverted.

(vii) The image formed in a plane mirror is virtual, erect, laterally inverted and is formed as far behind as the object is in front of a plane mirror.

3. Multiple images:

Kaleidoscope: A toy made by joining three plane mirror strips at an angle of 60°, such that coloured objects placed in it form beautiful hexagonal patterns.

4. White or Coloured:

(i) Dispersion: The phenomenon due to which white light splits into seven colours.

(ii) Spectrum: A band of seven colours formed on the screen, when white light splits into its constituent colours.

(iii) A prism breaks white light into seven colours to form spectrum, such that the red colour bends least, and the violet colour bends most.

5. Inside Our Eyes:

(i) Ciliary muscles: A ring of muscles which alter the focal length of the eye lens by expanding or contracting.

(ii) Cornea: A transparent tissue in front of the eye, which allows the light to pass through.Iris: A circular diaphragm which controls the amount of light entering the eye.

(iii) Retina: A hemispherical light-sensitive screen at the back of the eye on which image is formed.

(iv) In human eye, the retina acts as a sensitive screen. It is made of 70,000 nerve endings. On the retina yellow spot is most sensitive to light.

(v) The point where the optic nerve enters the eye has no nerve endings. It is insensitive to light and is called blind spot.

(vi) The three defects in the eye are:

(a) short-sightedness (myopia)

(b) long-sightedness (hypermetropia)

(vii) Temporary blindness is caused in the eye due to crystalline lens becoming opaque. It can be cured by replacing the crystalline lens by an artificial lens surgically.

6. Braille System:

Braille: A special script developed for visually challenged people for reading.

What is the Braille system? This video enables students to understand the history behind the Braille system and its importance for visually challenged people.

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