Wave Optics is a section that deals with behaviour of light and its wave characteristics. Wave topics are considered to be easy when it comes to scoring marks in NEET. One can easily score in this topic if prepared thoroughly. To help students prepare Wave Optics for NEET 2020, we are providing students with important topics, revision notes, and some tips and tricks to solve the questions.

• There will be 2 to 3 questions from optic waves topic in NEET 2020.
• The weightage of this topic is around 4% in physics section in NEET.
• The important sub topics of this topic are law of reflection, Huygens principles, Refraction of light, Constructive and destructive interferences etc. Check NEET Physics Syllabus

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We have also added quick notes on each sub topic so that you can quickly cover all the necessary topics. Along with it solved sample questions and previous year exam questions are also given in the article.

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Huygens Principle

According to Huygens Principle, at any instant, the wave front of a wave is defined as the locus of all particles in a medium distributed at the same time and in the same phase of vibration. According to this, each point on a wave front emits its own set of spherical waves (wavelets) and acts as a source of wave fronts. At any point of time, the locus or envelopes of wavelets in the forward direction gives the position of the new wave.

Examples of Huygens Principles:

1. Stone thrown in water creates circular waves (wave front waves) at that point dispersing in every direction. If locus of all wave fronts in same phase is joined, it appears as a sphere. It is known as primary wave front which further creates secondary wave fronts.

Sample Question

Question: State for which of the given characteristic dimensions, ray optics is valid?

1. Of the order 1mm
2. Much larger than the wavelength of the light
3. Of the same order as the wavelength of the light
4. Much smaller than the wavelength of the light

Answer: (2)

Question: Find the correct characteristic of the ether medium pervading entire universe according to Hugyen’s principle.

1. Not elastic
2. Highly elastic less dense
3. Less elastic and denser
4. Much heavier

Answer: (2)

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Laws of Reflection

It states that the incidence ray, the reflected ray and the normal to the reflecting surface at the point of incidence all lie on the same plane and that plane is perpendicular to the reflecting surface as shown in the figure below:

• It also states that the angle of incidence is always equal to the angle of reflection i.e, ∠i = ∠r.

Refraction of Light

Another important sub topic of Physics Wave Optics is Refraction of Light. Refraction of the light is the bending of light when it passes from one source to another. In other words, it is the phenomenon because of which, light when passing from one medium to another changes its velocity. This phenomenon is the reason behind prisms, lenses, rainbows etc. The concept of refraction of light is given in the figure below:

• Here, the incident ray, the reflected ray and the normal, all three lie on the same surface.
• Also, the sign of the angle between the incident ray and the normal is constant to the ration between the normal and the reflected ray, i.e, sin i/sin r = v1/v2 = 1µ2 = constant.
• Here v1 and v2 are velocities of sound in 1 and 2 medium respectively and 1µ2 is the refractive index of second medium to the first.

Sample Question

Question: Which of the given lights is least refracted by prism?

1. Red light
2. Violet light
3. Blue light
4. Green light

Answer: (1)

Question: For which of the following, the disruptive power will be maximum?

1. Crown glass
2. Flint glass
3. Mixture of glass
4. None of these

Answer: (2)

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Brewster Law

According to the Brewster Law, if an un polarized light is incident at polarizing angle (i) on an interface separating rarer medium from denser medium having refractive index “m”, such that µ = tan i, then reflected light in the rarer medium is completely polarized as shown in the figure below:

• Also in this case, reflected ray and refracted ray are perpendicular to each other.

Sample Question

Question: Find the polarization angle and angle of refraction respectively if a certain polarizer has a refractive index of 1.33?

1. ip= 53.06, ir= 36.94
2. ip= 53, ir= 37
3. ip = 55.01, ir= 34.99
4. ip= 53.1, ir= 36.9

Answer: (1)

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Interference

The modification obtained due to superposition of two or more waves in the distribution of light energy is called interference.

There are two type of interferences- Constructive Interference and Destructive Interference.

1. Constructive Interference- If two waves approaching each other line up their crest together, then it results in a wave of higher amplitude. It is known as constructive interference.

Conditions for constructive Interference:

• Path difference = (2n)λ/2
• Phase difference = (2n)π

2. Destructive Interference- When the crest of one wave meets the trough of another other wave then it results in a wave of low amplitude. It is known as Destructive interference.

Conditions for Destructive Interference:

• Path difference = (2n +1)λ/2
• Phase difference = (2n+1)π

Conditions for Interferences:

1. Both the sources should continuously emit waves of same wavelength and frequency.
2. Both the sources should be narrow and close to each other.
3. The amplitude of both the ways should be either equal or nearly equal.
4. Both the wave should be coherent one.

Sample Question

Question: What is the ratio of intensities at maxima and minima, if, ratio of amplitude of the two sources producing amplitude is 3:5?

1. 5:3
2. 25:9
3. 25:6
4. 16:1

Answer: (4)

Question: Which of the given concepts id responsible for colors on the soap bubble?

1. Interference
2. Polarization
3. Reflection
4. Refraction

Answer: (1)

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Diffraction

Diffraction refers to bending or spreading of waves when they encounter an object (a barrier) in their path. There are two classes of diffraction-

1. Fresnel Class of Diffraction, in this type of diffraction, source and screen are at finite distance from aperture.
2. Fraunhofer Class of Diffraction, it is a special case of Fresnel diffraction and in it, both source and screen are at infinite distance from the aperture.
3. Iθ = Im (sinα/α)2 where P is the value when lm represents the intensity at O.

Sample Question

Question: What is the wavelength of light used if Angular width of central maximum in the Fraunhoffer's diffraction pattern is measured and Slit is illuminated by the light of another wavelength, angular width decreases by 30%.

1. 4000 Å
2. 4500 Å
3. 4250 Å
4. 4200 Å

Answer: (4)

Question: In comparison with red light how does the diffraction band of blue light look?

1. No changes
2. Diffraction pattern becomes narrower
3. Diffraction pattern disappears
4. Diffraction pattern becomes broader

Answer: (2)

Important topics in Wave Optics

Wave Optics is one of the easy topics to score thus making it important when it comes to NEET exam. While Wave Optics in itself is an important topic to prepare for, it also has many important topics which are important from the point of vie of the preparation. Given below is the list of some important sub topics of Wave Optics that needs to prepare with due diligence:

1. Laws of Reflection
2. Refraction of Light
3. Hugyens Principles
4. Coherent and Incoherent Sources
5. Determination of Phase Difference
6. Constructive and Destructive Interferences
7. Conditions for interference
8. Magnification power of different microscopes
9. Polarization
10. Brewster Law
11. Diffraction
12. Young’s double slit experiment

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Previous Year Solved Papers

Question: What will be the ratio of resolving power of a Microscope for lights of wavelengths N1= 4000 Å and N2= 6000 Å?

1. 3:2
2. 9:4
3. 16:81
4. 8:27

Answer: (1)

Question: What will be the distance between first dark fringes on either side of the central bright fringe, if, a beam of light of 600 mm from a distant source falls on a single slit 1 mm wide and the resulting diffraction pattern is observed on a screen 2 m away?

1. 2.4mm
2. 1.2 cm
3. 2.4 cm
4. 1.2 mm

Answer: (1)

Question: What will be the refractive index of the medium is, if

1. It is found that a young’s double slit experiment is first performed in in air and then in a medium other than air.
2. 8th bright fringe in the medium lies where 5th dark fringe lies in the air.

Solution Options:

1. 78
2. 1.59
3. 1.25
4. 1.69

Answer: (1)

Question: What will be the ratio at intensity at minima to that at maxima if in a young’s double slit experiment the slit widths are in the ratio of 1:9?

1. 1/3
2. 1/9
3. 1/4
4. 1

Answer: (3)

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Some tips and tricks to solve Questions

Some tips and tricks to solve NEET 2020 Exams:

1. Start by attempting easy questions first on Exam Day

On exam day, first go through all the questions in the beginning and then start by solving the easy questions first, then moderate level questions and if time permits then difficult questions. Thus, you will be able to solve more questions in limited time.

2. Start giving mocks at the same time of exam

Since the date and time of the examination is available, you can start making your schedule according to the day of exam. Start giving mock tests at the same time at which you have exam on the D-Day. This will help your body and mind ready for the exam day.

3. Put Values while solving questions to narrow down your options

There will be some questions, where equations will be given as options. Solving these equations and checking each of them consumes a lots of time. Try solving them by putting numbers, this saves your time and narrows down your options too.

4. Read questions carefully

Use elimination method to solve the questions. On the exam day, read questions twice and then look at the options carefully. Since it is a MCQ paper, elimination method could be applied in some options and thus will save you a lot of time.

5. Do Not panic

Stay calm and stay focussed and do not panic if you are not able to solve any question. Breathe in, relax and move on to solve next one, and come back to it later on, if you have time.

Study Plan

Since NEET is just around the corner, most of you guys would be done with their syllabus, and would be doing revision these days. So here we provide you with a quick revision plan to help you cover all the necessary topics and some do’s and don’ts to help you within this short span of time:

• Make a checklist of all-important formulas, and revise/ read all of them regularly twice a day. You can revise them once after waking up in the morning and once at night before going to bed. It will take only 10-15 minutes but will give you a quick revision.
• Analysing mock tests is a very important part of your preparation. After attempting mock tests, analyse them and re-solve unattempted or wrong attempted questions.
• You can also try attempting previous year question papers and this year’s sample papers. This will help you know the difficulty level of the exam.
• Don’t try any new topics now, just revise previous topics thoroughly.
• Do not attempt any mock tests in the last week before the exam. Just go through your notes and revise difficult questions.
• Do not study for the last 2-3 days before the exam. Just revise the formulas and do anything that helps you relax- watch movies, web shows, anything that you like.