Physics Lab Quiz - Optics

1. Experiment E12 (L3): Refractive Index of the Material of a Prism using a Spectrometer

Q1: Why the spectrometer should be leveled before its use? Ans: Otherwise the position of the image will change with the position of the telescope, i.e., it will be different for different positions of the telescope.

Q2: What the Spectrometer and how are the conditions for obtaining a pure spectrum realized in it? Ans: The Spectrometer is used to measure the refractive index by finding the angle of the prism and the angle of minimum deviation. If the telescope of the spectrometer is replaced by a photographic camera, the instrument is transformed into a spectrograph. The measurement of refractive indices of a material for various spectral lines is provided by the spectrometer, which is a compact apparatus for obtaining a pure spectrum. It is equipped with a circular scale and a Vernier.

Q3: What are the main parts of a spectrometer? Ans: In its simplest form, it has the following main parts: i) the collimator, (ii) the telescope, and (iii) the prism table.

Q4: What is the function of the collimator? Ans: Collimator produces parallel beam of light.

Q5: Why do you set the telescope for parallel rays? Ans: If the incident rays are not parallel, i.e., if they are either divergent or convergent, the image focused for one position of the prism will be out of focus for other positions. If the incident rays are parallel, then both the object and the image of the prism will be at infinity, and the telescope, once focused for the image, will remain so for every position of the prism.

Q6: How do you place the prism when determining its angle by rotating the telescope? Ans: The prism table should be placed with its edge over the center of the prism table.

Q7: How do you place the prism when determining its angle by rotating the prism table? Ans: The prism table should be placed with its refracting edge at the center of the prism table when reading the angle by rotating the telescope.

Q8: How does the deviation change with color of light? Ans: Deviation is greater for violet than for red light. If the difference of wave length of light is considered, deviation will be different.

Q9: Why do you take sodium light and not white light? Ans: Sodium light is taken because it gives a single image of the slit and a single value of minimum deviation, while white light gives a spectrum and the value of minimum deviation is different for different wavelengths of light.

Q10: What is the condition of minimum deviation? Ans: At the minimum deviation position, the angle of incidence should be equal to the angle of emergence.

Q11: How does the deviation change with the color of light? Ans: Deviation increases with shorter wavelengths, e.g., it is greater for violet light than for red light.

Q12: How does the deviation of a ray vary with its angle of incidence? Ans: The deviation has becomes minimum at a particular angle of incidence, but it always increases when the angle of incidence is either increasing or decreasing than that at which the deviation becomes minimum.

Q13: What do you mean by monochromatic light? Is sodium light strictly monochromatic? Ans: Sodium light is strictly monochromatic. Light of a particular wavelength is called monochromatic. Sodium light is not exactly monochromatic, but it is considered a monochromatic light as it contains light of two wavelengths of values $5890^{\circ }\text{A}$ and $5896^{\circ }\text{A}$.

Q14: To measure the angle of a thick prism why the edge of the prism is placed at the center of the table? Ans: The error in the measurement of the angle of a thick prism is minimal by such placing.

Q15: Why readings of two Verniers are noted? Ans: Both Verniers should be read to avoid error due to the non-coincidence of the center of the circular scale with the axis of rotation of the telescope or the table.

Q16: What kind of an image does the telescope produce? Ans: A virtual image at infinity is produced by the telescope. A real diminished image is produced by the objective of the telescope while the eyepiece produces a virtual magnified image.

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3. Experiment E13 (L4): Wavelength of Sodium Light by measuring the Diameter of Newton’s Rings

Q1: What is the purpose of the traveling microscope? Ans: The original source states: See the text. (Self-Generated Answer based on theory): The traveling microscope is used for finding the length of very small objects. In this experiment, it is used to precisely measure the diameter of the Newton’s Rings.

Q2: What is the cause of Newton’s rings? Ans: The original source states: See the text. (Self-Generated Answer based on theory): Newton’s rings are caused by the interference between the light rays reflected from the lower surface of the lens and the upper surface of the optically plane glass plate. This interference results from the air film of varying thickness between the two surfaces.

Q3: Why are the rings circular? Ans: The original source states: See the text. (Self-Generated Answer based on theory): The rings are circular because the air film enclosed between the plane glass plate and the convex lens has a thickness that is constant for all points lying on a circle around the central point of contact.

Q4: Why does the center of the ring pattern appear dark in reflected light? Ans: The original source states: See the text. (Self-Generated Answer based on theory): The center is dark because the light reflecting off the lower surface of the lens (denser medium) undergoes a phase change of $\pi$ (or $180^{\circ }$) relative to the light reflected off the upper surface of the glass plate (rarer medium). At the point of contact, the air film thickness is zero ($t=0$), resulting in destructive interference and thus a dark spot.

Q5: Why is monochromatic light used? Ans: The original source states: See the text. (Self-Generated Answer based on theory): Monochromatic light (like Sodium light) is used because interference fringes (rings) formed by white light would overlap after the first few rings, making accurate measurement impossible due to the different wavelengths present.

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6. Experiment E14 (L5): Wavelength of Sodium Light and the Resolving Power of a Plane Diffraction Grating using Spectrometer

Q1: What is Diffraction of light? Ans: Diffraction is the bending of light from the edge of a slit and spreading of light waves or the illumination of the geometrical shadow of an object.

Q2: What is the difference between interference and diffraction? Ans: In interference, the resultant intensity at a point is the resultant of the superposition of two wave fronts coming from two coherent sources. But in diffraction phenomenon, the resultant intensity at a point is due to the superposition of wavelets from two parts of a single wavefront.

Q3: What is diffraction grating? Ans: A diffraction grating is a device which consists of a very large number of closely spaced parallel slits separated by equal opaque spacing.

Q4: What is transmission and reflection grating? Ans:

• Transmission grating: A plane diffraction grating consists of a very large number of narrow slits side by side. The slits are separated by opaque spaces. When a wave front is incident on a grating surface, light is transmitted through the slits and obstructed by the opaque portions. Such a grating is called the transmission grating.
• Reflection grating: If the lines are drawn on a silvered surface (plane or concave), then the light is reflected from the positions of the mirror in between any two lines, and such surfaces act as reflection gratings.

Q5: What is grating element, grating constant and corresponding points? Ans:

• Grating element: If $a$ be the width of a slit and $b$ be the width of an opaque space of a grating, then $(a+b)$ is called the grating element.
• Grating constant: The reciprocal of the grating element, $(1/(a+b))$, is known as the grating constant.
• Corresponding points: When two points in the consecutive free separated by a distance $(a+b)$, the grating element, and then these two points are known as corresponding points.

Q6: What happens if the number of rulings per cm, $N$, is increased or decreased? Ans: If $N$ is increased, the order number will be few but they will be separated by a large angle. If $N$ is decreased, the order number will be large, but separated by a small angle.

Q7: What is an absent spectrum? Ans: In diffraction grating the condition for principle maximum is $(a+b)\sin \theta =n\lambda$. Hence the first order spectrum is absent if $\lambda >a+b\sin \theta$ and $\sin \theta >1$. The second order spectrum is absent if $(a+b)<2\lambda$. In general if $(a+b)<n\lambda$, then the $n^{th}$ order spectrum will be absent.

Q8: What is the condition that the second order spectrum may be absent? Ans: In order that the $2^{nd}$ order spectrum may be absent, $\frac{a+b}{a}=2$, i.e., $a=b$. In this case the width of the slit is equal to the opaque spacing between any two consecutive slits. In this case the $2^{nd}$ order spectrum will be absent.

Q9: What happens if the ruled surface of the grating is directed towards the collimator? Ans: The incident rays will be first diffracted at the ruled surface and then will be again refracted by the glass surface. Thus the angle $\theta$ is not due to diffraction alone; it will be due to diffraction and refraction.

Q10: What do you mean by ghost line? Ans: If the rulings on a grating are not exactly equidistant or accurately parallel, then some additional lines appear near the main spectral lines. The additional lines are called ghost lines.

Q11: How does a grating spectrum differ from a prismatic spectrum? Ans: The angle of diffraction in the grating spectrum is proportional to $\lambda$ when $\theta$ is not very large. But in the case of the prismatic spectrum, the violet end is more drawn out than the red end. Hence the grating spectrum is a pure spectrum than a prismatic spectrum.

Q12: What is replica grating? Ans: Replica gratings are constructed from the original grating, possibly prepared by contact printing on a fine grained photo graphic plate. The original grating surface a thin layer of collodion solution is poured and the solution is allowed to harden. Then, the film of the collodion is removed from the grating surface and then fixed between two glass plates. This serves as a plane transmission grating. This is called replica grating.

Q13: What do you mean by resolving power? Ans: The ability of an optical instrument, expressed in numerical measure, to resolve the images of two nearby points is termed as its resolving power. Resolving power is the ability of the instrument to show nearby spectral lines as separate ones.

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9. Experiment E15 (L6): Wavelength of various Spectral Lines of Discharge Tube by using Spectrometer and a Plane Diffraction Grating

Q1: What is the grating element and grating constant? Ans: The original source states: Same as experiment number 5. (Self-Generated Answer based on E14/L5): The grating element is the sum of the width of a slit ($a$) and the width of an opaque space ($b$), or $(a+b)$. The grating constant is the reciprocal of the grating element, $1/(a+b)$.

Q2: How does the angle of deviation change with the wavelength of light? Ans: The original source states: Same as experiment number 5. (Self-Generated Answer based on E14/L5): In a grating spectrum, the angle of diffraction ($\theta$) is proportional to the wavelength ($\lambda$) when $\theta$ is not very large, according to the grating equation $n\lambda =(a+b)\sin \theta$.

Q3: What determines the order of the spectrum? Ans: The original source states: Same as experiment number 5. (Self-Generated Answer based on E14/L5): The order of the spectrum, denoted by $n$, corresponds to the integer value in the grating equation $n\lambda =(a+b)\sin \theta$. $n=1$ is the first order, $n=2$ is the second order, and so on.

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