## Monday, November 28, 2011

### CBSE Class 10- Chapter 11 - Human Eye and the Colourful World (Very short Q & A)

Human Eye and the Colourful World

Q1: Name a light sensitive part of the eye where image of an object is formed.

Q2: Name the muscle responsible for bringing change in the focal length of the eye lens ?

Q3: Name one defect of vision which cannot be corrected by any type of spectacle lenses ?

Q4: What is the nature of image formed on the retina of the eye ?

Q5: Name the transparent membrane through which light enters the eye.

Q6: What type of signals are generated & sent to the brain by light sensitive cells of retina?
Answer: Elctro-chemical Nerve signals are generated by rods and cones (photo sensitive cells of retina) and are sent to brain by optic nerve.

Q7: Which part of the human eye helps in changing the thickness of lens?

Q8: Which component of human eye controls the amount of light entering the eye?

Q9: Define the term power of accommodation in eye.
Answer: The ability of the eye lens to adjust its focal length is called accommodation

Q10: In which type of eye defect far point of eye gets reduced?

Q11: In which type of eye defect near point of eye becomes more then 25cm?

Q12: What is the function of optic nerve in human eye ?
Answer: Optic nerve sends the image formed on retina screen to the brain in the form of electro-chemical signals.

Q13: What type of lens is used for correcting hypermetropia ?

Q14: Which type of lens should be used to correct the presbyopia?

Q15: Define angle of deviation?
Answer: Because of multiple reflections or refractions, bending of light occurs. The angle at which emergent ray bends with respect to incident ray is called angle of  deviation. As shown in figure, it is denoted by a symbol δ (delta).
The angle of deviation is not the same for all the wavelenghts (colours) of light. Usually, for a prism the angle of deviation can be computed as:
δ = (μ -1 ) A, where A is angle of prism and μ is refractive index of prism material.

Q16: Who was the first person to obtain the spectrum of sunlight ?

Q17: Why does sky appear dark to an astronaut?
Answer: There are no dust particles in the upper atmosphere of the earth  and in the space because of which no scattering of light takes place. Therefore sky appears black (instead of blue) to an astronaut.

Q18: In visible spectrum, which colour has longest wavelength.
Answer: Red (that's why it deviates/scatters less).

Q19: Define angle of Prism.
Answer: The angle formed by two refracting surfaces (in case of prism) is called Angle of Prism. As shown in figure, the apex angle A is called angle of prism.
As shown in figure, if incident ray P is extended to L and emergent ray is back-traced, both meet at M. ∠LMQ is called angle of deviation. For practical purposes,
$i+i' -A$

Q20: Why danger marks or signals are in red colour?
Answer: Red having longest wavelength, scatters less and hence more visible.

Q21: Why do you take time to see objects when you enter a dim lighted room from outside in the sun?

Q22: What is the direction of rainbow formation? What is the position of red colour in a rainbow?

Q23: What is the basic cause of atmospheric refraction?
Answer: The different layers of the atmosphere have different optical densities which is the basic reason for atmospheric refraction experienced by the sunlight.

Q24 (Delhi CBSE Q Paper): The far point of a myopic person is 150 cm in front of the eye. Calculate the focal length and the power of lens to enable him see distant objects clearly.

Answer: Far point (v) = 150 cm
Using lens formula,  1/f = 1/v - 1/u
⇒ 1/f = (-1/150) - (-1/∞)
⇒ 1/f = -1/150
∴ f = -150cm
power =  -100/150 = -2/3 = -0.66 D