Chapter 9 · Light - Reflection and Refraction

NCERT Class 10 Science Chapter 9 Intext Questions (Page 150)

All 5 questions of NCERT Class 10 Science Chapter 9 Intext Questions (Page 150) — Light - Reflection and Refraction — in the book's order, each with its answer and a step-by-step solution.

1. Q1
   A ray of ​light travelling in air ​enters ​obliquely into water. Does the ​light ray bend towards ​the normal or away ​from the ​normal? Why?
   Show answer

   It bends towards the normal, because water is optically denser than air, so light slows down and $r<i$.

   The light ray bends **towards** the normal. Water is optically denser than air, because its refractive index ($1.33$) is greater than that of air ($1.0003$), so the light slows down on crossing the boundary. A ray travelling obliquely from a rarer medium into a denser one always turns towards the normal, and the angle of refraction is therefore smaller than the angle of incidence, $r<i$.

   Step-by-step solution →
2. Q2
   Light enters ​from air to glass ​having refractive index $1.50$. What ​is the ​speed of light in the ​glass? ​The speed of light in ​vacuum is $3\times 10^8$ m ​s${}^{-1}$.
   Show answer

   The speed of light in the glass is $v=2\times 10^8{\text{m s}}^{-1}$, two-thirds of its speed in vacuum.

   The speed of light in the glass is $2\times 10^8$ m s${}^{-1}$, since $v=c/n=(3\times 10^8)/1.50$. That is two-thirds of its speed in vacuum.

   Step-by-step solution →
3. Q3
   Find ​out, from ​Table 9.3, ​the medium ​having highest optical ​density. Also find ​the medium ​with lowest optical density.
   Show answer

   Diamond (refractive index 2.42) has the highest optical density and air (1.0003) the lowest.

   Diamond has the highest optical density, with a refractive index of $2.42$ — the largest value in Table 9.3. Air has the lowest optical density, with a refractive index of $1.0003$, the smallest value in the table. Light therefore travels slowest in diamond and fastest in air of all the media listed.

   Step-by-step solution →
4. Q4
   You are given kerosene, turpentine and water. In which ​of ​these does ​the ​light travel fastest? ​Use ​the information given ​in Table ​9.3.
   Show answer

   Light travels fastest in water, which has the smallest refractive index ($1.33$) of the three, so $v=c/n$ is greatest.

   Light travels fastest in water. Of the three liquids, water has the smallest refractive index ($1.33$, against $1.44$ for kerosene and $1.47$ for turpentine), and since $v=c/n$ the smallest refractive index goes with the greatest speed. The speed in water is about $2.26\times 10^8$ m s${}^{-1}$, against $2.08\times 10^8$ m s${}^{-1}$ in kerosene and $2.04\times 10^8$ m s${}^{-1}$ in turpentine.

   Step-by-step solution →
5. Q5
   The ​refractive ​index of diamond ​is $2.42$. ​What is the ​meaning of ​this statement?
   Show answer

   Light travels $2.42$ times faster in vacuum than in diamond, where its speed is $1.24\times 10^8{\text{m s}}^{-1}$.

   It means that the ratio of the speed of light in vacuum to the speed of light in diamond is $2.42$ — that is, light travels $2.42$ times faster in vacuum (or air) than it does inside diamond. Its speed in diamond is therefore $v=c/2.42=(3\times 10^8)/2.42=1.24\times 10^8$ m s${}^{-1}$. Because $2.42$ is the largest refractive index in Table 9.3, it also means diamond is the optically densest of those media, and bends a ray entering it obliquely from air more sharply towards the normal than any of them.

   Step-by-step solution →