A camera lens is marked `f//1.8`. What is the meaning of this mark?

The optician's prescription for a spectacle lens is marked +0.5 D. What is the, a) nature of spectacle lens? b) Focal length of spectacle lens?

An object placed 4 cm in front of a converging lens produces a real image 12 cm from the lens. a) What is the magnification of the image? b) What is the focal length of the lens? Draw a ray diagram to show the formation of image. Mark clearly F and 2F in the diagram.

Students of three sections of a class, having 30, 30 and 40 students appeared for a test of 100 marks. The arithmetic means of the marks of the three sections are 72.2, 69.0 and 64.1 in that order. What is the arithmetic mean of the marks of all the students of the three sections?

Lens Camera

The data below shows the number of students in class A having secured different marks. What is the mean marks of class A?

Students of two schools appeared for a common test carrying 100 marks. The arthmetic means of their marks for school I and II are 82 and 86 respectively, If the number of students of school II is 1.5 times the number of students of school I, what is the arithmetic mean of the marks of all the students, of both the school ?