(a) Describe the working principle of cellular networks. How do mobile devices maintain connectivity while moving between cells? (b) Using the photoelectric effect, explain the particle nature of electromagnetic radiation. How did experimental observations challenge Classical Wave Theory and lead to the development of quantum mechanics? (c) List the major social media platforms used for business and communication. How do they contribute to digital marketing? Explain. (d) Explain how the GPS system determines the position of a receiver using satellites.

(a) Cellular networks

Working principle: a service area is divided into many small areas called cells, each served by a base station (cell tower). Phones communicate with the nearest base station by radio waves. Base stations connect to the operator's core network, which routes calls and data to other networks and to the internet. Non-neighbouring cells can reuse the same frequencies (frequency reuse), which allows a limited radio spectrum to serve many users.

Maintaining connectivity while moving: the phone and network constantly measure the signal quality of the serving cell and neighbouring cells. As a user moves away and the signal weakens, the network carries out a handover (handoff), transferring the connection to a stronger neighbouring cell, with usually no noticeable interruption. The network also tracks the phone's location through location updates, so that incoming calls can reach it. Roaming extends the service to other operators' networks.

(b) The photoelectric effect

The photoelectric effect is the emission of electrons from a metal surface when light of suitable frequency falls on it.

Observations that challenged Classical Wave Theory

Threshold frequency: no electrons are emitted below a certain frequency, however intense the light. Wave theory predicted that any frequency would work if bright enough. Kinetic energy depends on frequency: the maximum electron energy rises with frequency, and not with intensity. Wave theory predicted the opposite. Intensity affects number: brighter light releases more electrons, but not faster ones. Instant emission: electrons emerge almost immediately, even in dim light. Wave theory predicted a delay while energy accumulated.

Einstein's explanation (1905): building on Planck's quantum idea, Einstein proposed that light consists of packets of energy called photons, each with energy E = hf. One photon transfers all its energy to one electron. If this exceeds the metal's work function (φ), the electron escapes with maximum energy KE = hf − φ. This explains every observation, and it shows the particle nature of electromagnetic radiation.

Path to quantum mechanics: this work, confirmed by Millikan, showed that energy exchange is quantised, and led to the idea of wave-particle duality. It was followed by Bohr's atomic model, de Broglie's matter waves, and the quantum mechanics of Heisenberg and Schrödinger. Einstein received the 1921 Nobel Prize for this explanation.

(c) Social media and digital marketing

Major platforms: Facebook, Instagram, WhatsApp, LinkedIn, X (Twitter), YouTube, TikTok, Pinterest and Snapchat.

Contribution to digital marketing

Brand awareness: businesses can reach large audiences at a relatively low cost. Targeted advertising: ads can be directed by location, age and interests. Customer engagement: comments and messages build direct relationships and offer customer service, as WhatsApp does for many small businesses in Pakistan. Content marketing: videos, posts and live streams attract and inform customers. Influencer marketing: trusted personalities promote products to their followers. Social commerce: shops and links lead directly to sales. Analytics: data on reach and conversion allow campaigns to be measured and improved. Professional networking: LinkedIn supports business-to-business marketing and recruitment.

(d) How GPS determines position

Each GPS satellite broadcasts a signal giving its exact position and the time from its atomic clocks. The steps are as follows.

1. Signal reception: the receiver picks up signals from several satellites. 2. Travel time: it compares the time of sending with the time of arrival. 3. Distance: distance = speed of light × travel time. Each distance places the receiver on a sphere centred on that satellite. 4. Trilateration: the intersection of spheres from several satellites gives the position. 5. Clock correction: the receiver's clock is much less accurate, so a fourth satellite is required to solve for this time error along with latitude, longitude and altitude. 6. Refinement: corrections are made for atmospheric delay and relativity, and using more satellites improves accuracy.