CBSE Class 10 Science 2017 · Paper 31/3/C · Question 23

Laws of refraction (obeyed at each surface of the lens): (1) The incident ray, the refracted ray and the normal to the interface at the point of incidence all lie in the same plane. (2) The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for light of a given colour and a given pair of media: $\frac{\sin i}{\sin r}=\text{constant}$ (Snell's law). Ray diagram: a ray from the top $A$ of the object, parallel to the principal axis, diverges after refraction as if it came from the focus $F_1$; a ray through the optical centre $O$ goes on undeviated. The backward extension of the first ray meets the second ray at $A^{\prime }$, between $F_1$ and $O$, giving the image $A^{\prime }{B}^{\prime }$ (see the figure). Characteristics (any two): the image is virtual and erect; it is diminished; it is formed between $O$ and $F_1$, on the same side of the lens as the object. Magnification $+0.5$: the positive sign means that the image is erect (and virtual); $0.5$ means that the image is half the size of the object, $h^{\prime }=0.5h$.