Question 2 - Case Based Questions (MCQ) - Chapter 11 Class 10 - Human Eye and Colourful World

Read the passage on the human eye and answer any  four questions from (a) to (e). 
Eye is a natural optical device by which humans  could see objects around him. It forms an inverted,  real image on a light sensitive surface. It works on  the phenomenon of refraction of light through a  natural convex lens. However, Meena was unable to see clearly the words written on the black board  placed at a distance of approximately 3 m from her.  Her mother discussed the same with the doctor.  Doctor explained to her about this defect of vision  and its correction.  

(a) The human eye forms the image of the object at its  

(i) Cornea 

(ii) Iris 

(iii) Pupil 

(iv) Retina  

Answer

Human eye forms the image at it’s retina.

So, the correct answer is (ii)

(b) What kind of defect Meena is suffering from?  

(i) Myopia 

(ii) Hypermetropia  

(iii) Astigmatism 

(iv) Malnutrition  

Answer

Given that

• Meena was unable to see clearly the words written on the black board  placed at a distance of approximately 3 m from her.
• So, she could see the  nearby objects clearly but was unable to see the far  off objects . 
• Thus, she was suffering from Myopia  

So, the correct answer is (ii)

(c) The possible cause this defect is  

(i) eyeball is of same size  

(ii) eyeball becomes long  

(iii) eyeball becomes small  

(iv) None of the above  

Answer

• Myopia is caused due to excessive curvature of the eye lens
• Thus, images are formed in front of the retina

So, the correct answer is (ii) - eyeball becomes long  

(d) The closest distance up to which a person can see without any strain in the eyes  

(i) 35 cm 

(ii) 15 cm  

(iii) 5 cm 

(iv) 25 cm  

Answer

• A normal eye is not able to see   distinctly the objects placed closer than 25 cm ,  without putting any strain on the eye. 
• This is  because the ciliary muscles of the eyes are unable to  contract beyond a certain limit .. Hence 25cm is the minimum focal length for the human eye. 
• If the objects are  placed at a distance less than 25 cm from the eye,  then the objects appear blurred because light rays   coming from the object meet beyond the retina .  

So, the correct answer is (iv) - 25 cm

(e) The defective eye of a person has near point 0.5  m and far point as 3 m. The power of both lens required for reading purpose and seeing far off  objects is  

(i) 0.75 D and +4 D 

(ii) +2D and – 1/3 D  

(iii) – 2.5D and + 1/8D 

(iv) 0.85 D and – 2 D 

Answer

When the far point of a person shifts from infinity to somewhere nearer, it is called nearsightedness or Myopia .

• This happens when the ciliary muscles in the eye are unable to contract further, and the lens remains thick. 
• The image is formed in front of the retina instead of on it. 
• This is corrected by using a diverging lens  
• It increases the total focal length of the lens+eye combination, making the image on the retina. 

When the near point of a person shifts from 25cm to somewhere farther,  it is called farsightedness or Hypermetropia. 

• This happens when the ciliary muscles in the eye are unable to expand further, and the lens remains thin.
• The image is formed behind the retina instead of on it. 
• This is corrected by using a converging lens  
• It decreases the total focal length of the lens+eye combination, making the image on the retina. 

Let the object distance = u  

Image distance = v  

Thus, the correct answer is (ii) +2D and – 1/3 D